'hard-on' definitions:

Definition of 'hard-on'

(from WordNet)
noun
An erect penis [syn: erection, hard-on]

Definition of 'hard-on'

From: GCIDE
  • hard-on \hard-on\ n. An erect penis; a penile erection. [slang or vulgar]
  • Syn: erection. [WordNet 1.5]