As in, “He’s got a big pair of guns.”
What’s that all about? What’s the origin of that expression? Is it a play on the use of arms to mean weapons?
And does it refer to the biceps, or the whole arms, or what?
[gratuitous complaint about the stupidity of the usage omitted]
I don’t know about that, but once I saw a woman with a nice pair of thirty-eights! 
This is my rifle;
This is my gun.
One is for fighting;
One is for fun.
“He’s got a big pair of guns.” 
Nobody’s ever heard this? I first encountered it on a TV show, some guy bragging about the size of his arms.
I heard it again today on the radio, some DJ talking about Vin Diesel and his arms, “Yeah, he’s got a big pair of guns.” He was talking to some chick who was drooling over guys’ arms.
Any possibility he was talking about gams = legs?
Well, guns have been known as firearms for a long time, so it doesn’t take much imagination to switch the terms and call your arms guns. I’ve heard it mainly in a baseball context, where a player is said to have a gun (or a cannon) for an arm, reflecting the fact that he can throw a ball fast and straight like a bullet out of a gun.
Having arms described as “guns” was not unknown in northeastern Wisconsin, at least in the late 80’s to early 90’s. It wasn’t used all that frequently, however. I can recall several incidents of this, usually involving cheerleaders and games that required arm strength, like swimming or volleyball.
I can remember at least once incident where someone from the suburbs of Chicago (Hinsdale, IIRC) being very confused at the term. I never thought that it might not be a particularly widespread usage, as confusing someone from the suburbs of Chicago is not a particularly difficult feat.
It would be similar to the “gams = legs” slang, except that it’s more current. Usually when someone used “gams”, it was with an ironic, retro linguistic inflection, whereas guns was a more current term. However, with the current rate of social recycling, it’s possible that “guns” currently seems as outdated as “gams” did then.
Wow, so nobody has a clue? At least I’m not the only one totally out of it.
Thanks for the link, Geobabe. That at least clarifies that they mean the biceps. The why is still a mystery, unless my WAG is lucky.
Still no answer as to why?
I’m really surprised that some people don’t seem to be familiar with “guns” being slang for big arms. I don’t have an answer for the OP, but a common line when I was in high school went like this:
Guy: “Do you have tickets to the show?”
Gal: “What show?”
Guy (flexing): “The gun show!”
The baseball origin is probably correct, according to Lighter. As early as 1929 the NYT was writing "A player’s arm is his gun or his wing. A good gun means that the possessor has a stong arm.
Then a quote from something in 1973 "Guns–the biceps and triceps part of the arm.
But, outside of baseball, I can’t remember hearing it in the 1950’s-60’s in Virginia. Not referring to biceps, etc.