means "incredibly immense" in several languages
The following is from: http://segal.org/name/index.html
Origin of the name Segal
"Segal" was used as a surname by some Levites in Europe, beginning in recent centuries when surnames replaced patronyms. However, "Segal" has been used for much longer as a title for Levites, predating the use of surnames.
The correlation between "Segal" and Levite has been blurred a bit over the centuries. In English-speaking countries, Segal has been blurred with "Siegel" in the past century. In English, Segal is pronounced as "Seegl", sounding the same as the more common unrelated name Siegel, used by non-Levites. Some families have different spellings of the surname in different branches of the family as a result of blurring of distinctions between Segal and Siegel. This blurring is a recent phenomenon since previously the names were kept distinct by the different pronunciation of Segal and Siegel in European and Semitic languages. In Hebrew, Segal is spelled "Samech Gimmel Lummid" and pronounced "sehgull", while Siegel is spelled "Samech Yood Gimmel Lummid" and pronounced "seegull", a distinction also found in European languages other than English. However, not all Segals are Levites due to the spelling changes and other reasons such as adoptions or name changes.
The details of the origin of the name Segal are not clear. The best evidence suggests that Segal is the acronym of the Hebrew phrase "SeGan Leviyyah", a designation applied to Levites many centuries before it was used as a surname. Using pairs of letters from "SeGan Leviyyah" to form the acronym Segal may seem troubling to speakers of English, but this is the format typically used for acronyms in Hebrew.
The earliest use of this designation that we are aware of is by Rabbi Isaac ben Eliezer (d. 1070 CE), one of the great "scholars of Worms" and a teacher of Rashi. According to the Encyclopedia Judaica, Rabbi Isaac ben Eliezer was "known as segan Leviyyah-meaning a Levite", and in Rashi's commentary on the Talmud, Rabbi Isaac is referred to as "Leviyyah". The acronym of Segal is used in the introduction to "Sefer Maharil". The editor, the Maharil's disciple Rabbi Zalman, writes of the Maharil (Rabbi Jacob ben Moses Levi Moelin, Mainz 1360-1427): "he is noted in the gates by his designation 'MaHa"R Jacob Moelin'; however I have included as his epithet 'MaHaR"I Sega"l' because he was from the tribe of the Levites".
It is not clear why "SeGan Leviyyah" would be used as a title instead of the simpler designation "HaLevi". "SeGan" is typically used in Hebrew to mean "deputy", rendering the designation as "Levitical deputy". Intriguingly, the Alcalay dictionary gives an archaic definition of the word "SeGan" as "(formerly) deputy to the High Priest". This archaic definition would then render Segal as "Levitical Deputy to the High Priest", which makes more sense. However, there were many subtleties to the priestly and Levitical designations, including the term "HaKohanim HaLeviim" used in the bible, so the designation Segal may refer to some subtle hierarchy relationship.
Beth Hatefusoth (Museum of the Jewish Diaspora) has the following explanation of the name Segal:
An abbreviation of the Hebrew "segan-leviyyah", which means "prince of the Levites" or "assistant of the high priest", Segal was originally a function and title which became a family name. Some of its forms include the customary Slavonic endings indicating descent from a male ancestor: -ov/off, -ic/ici and their Germanized spellings -itz/itsch. Changes in vocalization and the trend of adjusting to the languages of the ethnic minorities among whom Jews were living in the diaspora, produced variants such as Sagal and Sigal but also Segel and Siegel ("sail" and "seal" in German), the French Segalot (derived from the Polish/German Segalovitz) and the Hungarian and Polish Szegal.
A remarkable combination is represented by the compound Segelersigler ("sailor-sealer" in German), comprising two derivatives of Segal.
Apparently "meaningless" names in this group such as Ziegelbaum (in German "brick tree"), Zygelberg (at first sight an unusual spelling of the German Ziegelberg, that is "brick mountain"), and Segelbaum (in German "sail tree"), actually veil significant symbols deeply embedded in Jewish tradition.
Distinguished bearers of the family name Segal include the 18th century artist, Hayim ben Isaac Segal, who created the magnificent interior of the wooden synagogue in Moghilev, White Russia and the Rumanian-born German painter, Arthur Segal (1875-1944).
In Lithuanian, suffixes are added to names to denote marital and gender status. Accordingly, a male would be referred to as Segalis, an unmarried female as Segalyte, and a married woman as Segaliene.
Another example of Segal variants is the name Chagall in France. The painter Marc Chagall was a Levite.
Some similar names have entirely different origins from Levite Segals. Some people in India spell their name Segal, pronounced "sehgull". This spelling is a variant of the common family name in India of "Sehgal". The similarity to the Hebrew name seems to be just a coincidence.
There are other explanations for the name Segal, but these may be variants of the "SeGan Leviyyah" explanation. One is "SeGan LaKohen" or "SeGan LeKehuna'", meaning "Deputy to the Priest". This makes a bit less sense in terms of the acronym, and appears to state the Levitical role a bit more clearly. However, the existence of earlier sources supporting the "SeGan Leviyyah" explanation make "SeGan Leviyyah" more likely to be the true origin. But the "Sgan L'Kohen" explanation also has historical support in that members of the Landau family who are descended from the "Nodah B'Yehudah" would write their names as "Segal Landau", because as Levites they were "Sgan L'Kohen".
A third explanation relates to the Hebrew word "segol", which means the color violet, which may have been a reference to the Levite color. This explanation may be consistent with the other explanations, having been intended as something of a pun in addition to the acronyms.
If you have additional information that would allow one to distinguish between these possibilities, know of other early mentions of the name Segal, or if you have additional information about the origin of the name Segal, please send e-mail to the Segal.org Webmaster who will attempt to make sense of this.