Noun

1. An abortion, or a premature birth.
   • 1646: Wherein notwithstanding, we should rather rely upon the urine in a castling’s bladder — Sir Thomas Browne, Pseudodoxia Epidemica, Book II, ch 5
2. The second or third swarm of bees which leaves a hive in a season.

Noun

1. A move in which the king moves two squares towards a rook, and the rook moves to the other side of the king; the action of the verb to castle.