Archaic, literary euphemism for a thief, originally from a Chinese anecdote. Not used in modern everyday speech.
In classical literature, a thief is sometimes called a 'gentleman on the beam'.
Rare, humorous or euphemistic term for a mouse, likening it to a little thief on the beam. Very uncommon.
'Gentleman on the beam' is also occasionally used as a euphemism for a mouse.