at a heat—(coll.) at a single intense effort or action:
- The new articles having been “thrown off at a heat,” stood particularly in want of re-revision.
in heat—(of animals) in the state of sexual excitement during the breeding season (also: on heat):
- “I’m no bitch in heat,” she said between tight teeth, “take your paws off me.”
See also: at a white heat / in a dead heat.