From "How I Met Your Mother: Slap Bet (#2.9)"
[after Ted tells the gang that Robin was married in Canada
] Yeah, well, you still shouldn't have told us. I mean, what kind of boyfriend are you?
See, that's just it. I'm not the boyfriend, I'm the mistress. No, not the mistress. The mastress. Master. What do you call it?
I'm pretty sure we're gonna call is mistress.