Discussion Forum—A Way with Words, a fun radio show and podcast about language

On "McHale's Navy", Ernest Borgnine was referring to his gig.  I thought only those who commanded ships had gigs, not those like Quenton McHale, who commands a boat.  The dictionary I checked was not clear on it; they said a gig was the boat of a commander.

They also said there were five different gigs.  Gig [5] comes from the SI prefix for "a billion".  Gig[4] is an engagemrent for a musician, origin uncertain. Gig[3] is a demerit, origin uncertain. A frog sticker is a gig[2], from fizgig. Gig[1] includes a light 2-wheel horse carriage, or a small fast boat, something that whirls, a gig mill to raise nap, or (obsolete) a whirligig, originating from a middle english word for a flighty girl.  I suppose giggle is related, altyhough I didn't look it up.

My question: Although gig[5] and gig[3] have different origins, why are there 5 different definitions for gig[1], while gig[3] and gig[4] are considered to be different words?  It seems to me that someone gigging for frogs does the same thing as a headmaster gigging a strudent, and a musician snags a performance the same as one snags frogs with a gig. 

And a fizgig is a firework that makes a loud rushing-wind sound as it spins. Supposedly a fishgig is related to fizga, which is spanish for harpoon.

Seems to me that with the exception of giga- (which comes from the Greek for giant), they all seem to come from a silly girl, giggling, spinning around, trying to catch a fellow.  After all, it's not like English borrow from other languages, and they don't borrow back.  Borders have always been fairly fluid, and familiarity breeds. 

But if one carried that idea too far, we might end up with a dictionary that was One Degree of Kevin Bacon.

So how do they decide how many different words gig (and similar words) constitutes?