Does the average American know its meaning? Is it used commonly in the spoken language? What connotations does it have?
Is it gender specific?
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To take your questions in order, starting from the title question:
If you modify the parameters in NGrams, you'll see that limiting to the British or American corpus doesn't change the trend much.
As an AmE speaker it sounds very Wodehousian to me, like some old guy with a monocle drinking a g and t with an old rugby mate from public school.
If it were spoken in context, an American would easily figure out that it is something vaguely like 'friend or pal or buddy or whatever'. But it sounds very high class British to Americans.
The great majority of Americans don't have it in their production lexicon. Only a very very few Masterpiece Theater watchers (exposed to older British media) might at a stretch use it tongue-in-cheek.
It is just as gendered as buddy/pal/mate which means it is used a) mostly b) by men, talking to c) men but d) that's not a hard rule. (that is, -if- it were to be used at all in the US)