"Gig
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)."

from

Wordorigins.org: Letter G