bucking bronco

Short definition from Wiktionary

Noun

1. 1
   A wild or untamed horse, characteristically difficult to ride.
2. 2
   A spirited or uncontrollable person.
3. 3
   A mechanical bull in the form of a horse.
4. 4
   A supposed sexual position that involves the man telling the woman something disagreeable during sex (e.g. that he has a sexually transmitted disease, or prefers another woman), causing her to struggle or lash out.