other woman 
noun (the other woman)
The lover of a married or similarly attached man: her husband left the matrimonial home to live with the other woman...
- They had both two-timed her, and then married the other woman in both cases.
- You want him to stop seeing the other woman, but she may want you to do the same.
- His dad left his family when the other woman came to calling and left his mom to raise six kids and work.
- between you and me and the wall
- be twice the man/woman (that someone is)
- betwixt
- betwixt and between
- be two a penny
- be two (or ten) a penny
- beun
- be unable to believe one's eyes (or ears)
- be unable to hear oneself think
- be unable to (or be hardly able to) believe one's luck
- be unable to punch one's way out of a paper bag
- be unable to see the wood (or the forest) for the trees
- be under no illusion (or illusions)
- be under the illusion that
- be up shit creek
- be up shit creek (also be up shit creek without a paddle)
- be up the creek
- be up with the lark
- beurre blanc
- beurre manié
- beurre noir
- beurre noisette
- beurré
- Beuthen
- Beuys, Joseph