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Basically having a wank, jerk, or male masturbation.

Your erect penis represents the banjo "shaft" and the hairs represent the strings. Your ball-bag/nuts/scrotum representing the resonant body of the banjo.

Play away and try not to get too carried away or else you might.... Ooops.... Sorry... I've just cum.
To quote the words of a famous song:

"Bum-titty-titty, bum-titty-titty, play the hairy banjo !"
by Losso of the shire August 21, 2011