To indulge/vent an urge that society does not approve of -- this can be fighting, partying, drinking, having sex, smoking pot... whatever. It implies that there are other times when you're a "respectable citizen" and repress the urge -- full-time party-animals are not getting their ya-yas out because they always act that way.
Example for ya-yas, to get out:
"I have people flipping me off all the time but because I'm from New York, I get my ya-yas out that way and I get to flip 'em back." (quote from BBC News article "Environment drives Hummer vs Hybrid row" 1 June 2004)
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