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1.
Where a boy/man moves onto the 'bases' on relationships frequently or too quickly. eg first base (kiss) second base (oral) so on.

Preferably moving quite frequently, onto many girls, doing sexual activities. Known also as a player.
Harry: I got sucked off by Kirsten yesterday night, but the day before fingered Katy in a bush!

Martin: Harry you baseman!

Or

Cindy: Did you hear that Harry got head off Kirsten but fingered Katy and tried to have sex with Marie!

Laura: Ew, OMG, what a baseman
by Hahayoubaseman November 03, 2013