Here's the entry for "gig" from http://www.etymonline.com/index.php?term=gig

gig (1)
"light carriage, small boat," 1790, perhaps, on notion of bouncing, from M.E. ghyg "spinning top" (in whyrlegyg, 1440), also "giddy girl" (giglet), from O.N. geiga "turn sideways," or Dan. gig "spinning top."

gig (2)
"job," first used by jazz musicians, attested from 1915 but said to have been in use c.1905; of uncertain origin.

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Here's a post from a member of an online musicians' forum:

"Found this on a website about word origins:
Gig is an interesting word with a variety of etymologically unrelated senses.

The oldest sense is that of a top or other whirling object. It dates to c. 1440. The origin is unknown. The sense of top is the source of some other senses, such as a giddy or flighty person, fun, merriment, and a whim.

The sense of a light, two-wheeled carriage dates to 1791. This sense is a transference from the earlier sense. The motion of the carriage and its tendency to upset are not unlike that of a top. Also related is the sense of gig meaning a ship's boat. This nautical sense dates to 1790.

Gig can also mean a spear or harpoon, and it is a verb meaning to spear or stab, as in Gig'em Aggies. This sense originally comes from the Spanish word for harpoon, fisga. The Spanish word appears in English in 1565 as fisgig. An alternate spelling is fizgig and the word is also folk-etymologized as fishgig, a harpoon for stabbing fish. The clipped form gig appears in 1722.

This brings us to the most common sense, that of a musician's engagement or job. The musical sense dates to 1926 and first arose as jazz slang in the US. But the origin is not in music. The use of gig to mean a non-musical job or occupation dates to 1908, and the sense of a business affair or event is a year older than that. The origin is unknown, but it may come from the slang term gag. This dates to 1890 and means business method, practice, or behavior. All these sense are American slang usages.

These last senses may be from, or be influenced by, an obsolete sense of gig. This sense of gig is a type of bet in a numbers game. It dates to 1847 and is an arbitrary use of the sense of gig as a carriage (a horse is another type of bet).

Bill Dinwiddie

http://www.trombone.org/trombone-l/archives/200605/060523.txt

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Bill Dinwiddie didn't document the name of the online etymology resource that he used. It's possible it might have been http://home.netcom.com/~mrlucky/gig.html :

"What's the origin of the word "GIG"?

Longtime audio guy Peter Stefan asked about the derivation of the word "gig". He was referring to the definition, "a job usu. for a specified time; esp.: an entertainer's engagement. (This def courtesy of Merriam-Webster's Collegiate® Dictionary).

The first historical reference for this meaning is documented as occuring in 1926, both in Webster's and the OED. Both assert that the origin is unknown.

www.phoenix.net/~melanie/et_e-g.htm#carnival is an archive of "Your Etymology Questions", wherein it is suggested, "…gig-a musical performance, likely comes from French gigue 'a ball or dance', from Middle French giguer 'to dance'. Jig (the verb) likely comes from the same source."

Now, no dictionary I checked (six or so) gives the word gig this definition, let alone this derivation, so I place little credence in this source.

Gig is truly a hard-working word. The earliest meaning of gig is Sumerian, in which it means black or dark. If you go back just a few hundred years, it refers to the vagina AND the asshole. Later in history, definitions include: something that whirls or is whirled; a 3-digit selection in a numbers game; a person of odd or grotesque appearance; a long light ship; a rowboat designed for speed rather than for work; a light two-wheeled one-horse carriage; a pronged spear for catching fish; a military demerit…and that's just the nouns!

The etymology for this slew of meanings varies, but the specific meaning about which Peter queried seems to have no identified derivation. Sorry Pete, I guess I blew this gig. No, not THAT kind of gig! What a mind!"