Most of us grew up with the idea that the complete and correct name for the month following Tishrei is Cheshvan, only some people nicknamed it Mar Cheshvan because it is bitter (Hebrew: mar) due to its lack of holidays.
Turns out this is not true. Here's Rabbi Ari Zivitofsky with the full story:
[The word] Marcheshvan is probably derived from its location in the calendar. In Akkadian (Babylonian/Assyrian), “w” (vav) and “m” (mem) sounds can inter- change. As a result, Marcheshvan which is from the two words “m’rach” and “shvan,” would have been “warh” and “shman,” in Akkadian, corresponding to the Hebrew “yerech shmi- ni,” thus “eighth month.” In the Yemenite tradition, the name of the month is pronounced Marach- sha’wan, not Mar-cheshvan as in theAshkenazic tradition, and this would seem to preserve a greater fidelity to the original.Facinating, no? Legions of Jews call the month "Cheshvan," when the truth is this is just a silly mistake. Also true, is that names of the months themselves come from the Babylonian language. For example, Tishrei is related to the Akkadian word for "begining;" Iyyar to the word for "blossom" and so on.
Older sources attest to the name as being the longer name Marcheshvan/ M’rachshwan (as opposed to just Cheshvan). When the eighth monthis mentioned in the Mishnah and Talmud, it is referred to as Marcheshvan. A few examples include: Taanit 1:3,4; Pesachim94b;and Rosh Hashanah7a; 11b. Throughout all of Rashi’s Biblical andTalmudic commentary, he also refers to the month as Marcheshvan. The Rambam and Ibn Ezra (commentary to Leviticus 25:9) also use the complete name
[Boruch she kevanti: S. had it first. I found his post after I wrote mine.]
[Related: S. also suggests that our custom of singing wedding songs and throwing up an ad hoc bridal canopy during the Torah reading on Simchas Torah isbased ona similar mistake. He provides credible evidence that the last aliya was originally called "chatam Torah" ie the "sealing of the Torah" and in time this evolved into "chatan Torah" ie: Bridegroom of the torah.