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