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Five on one basketball is the act of masturbating. Since your hand has five fingers(generally) and you use your hand to masturbate(usually), the term was coined.
Man, when I'm stressed out, I get in a good game of

"five on one basketball" and I feel much better.

"Hey, did you and that broad have sex last night?"
"Nah, she got me all worked up and left, so I got in a good session of 'five one one basketball' to cure my blue balls"
by Headuardo December 15, 2011