In rowing (Sweeps, not sculls), it is the ability to row on both the port and starboard side.
Because scullers row with two oars all the
time, they generally tend to have better technique, and are more inclined to be bisweptual should they ever
switch over to sweeps.
It's cleverly worded to sound like "bisexual", so those uninitiated to rowing
will think you're mysterious and rebellious.
As far as being a hackneyed expression, it's the equivalent of saying that you're
bi-sacksual.
Coach: Tom, Pete is sick, and can't row today. I know you're a starboard, but he's port, and we need someone to make the change.
Tom: Sure coach, I'll do it, after all, I'm bisweptual.
Coach:
Ah, I see, so you could say that you row both ways?
Tom:...that's right Coach, anything for you.
Coach: Alright,
well today you're going to be stroking it also. I expect we're up against some
stiff competition.
Tom: Well I'm sure we can beat them off, if not, I'll just jack up the stroke rate.
Coach: You're sure you can handle that?
Tom:
Hell, they'll be so scared of me, they won't
even come.
Coach: Ok, this is getting too gay.