second fiddle
an inferior position (see also play second fiddle)
He hated the idea of playing second fiddle to his rival.
second fiddle
Ones back-up girlfriend/boyfriend.
No song is as sad as the song of the second fiddle. (OR) She was being played as his second fiddle.
second fiddle
n. The second girl in a threesome who is getting fingered but not screwed.
My girlfriend and I had a threesome, but the one rule was that the other girl had to be second fiddle.