The way someone looks or feels when they've had a hard time of it. From a horseman's term, when someone has not taken care of a horse after a hard day.
A: There's this chick and I know she really wants my dick.
B: All right, man!
A: Problem is, she's dating one of my best friends B: Damn, sounds like you're between a rock and a hard-on.
Some of David Letterman's female employees were caught between a rock and a hard place; They either had to sleep with a cranky and humorless old man or risk getting passed over for promotions by women that were.
When my wife walked out because I wouldn't help her becuase I thought what she was doing was wrong. She gave me the choice to change my mind and follow her, if I really loved her, or sit at home and never see her again. I was really stuck between a rock and a hard place.