A player, a man who sleeps with many women or has done so in the past.
Origin: the poem "Birches" by Robert Frost where there is a line that goes "one could do worse than be a swinger of birches" but someone swapped birch for bitch and a new slang phrase was born.
Greg is a total swinger of bitches, he slept with Tara and Meghan in the one night. What the hell?