Definition of Q-boat in US English:

Q-boat

(also Q boat)

noun

historical
  • An armed and camouflaged merchant vessel used as a decoy or to destroy submarines (chiefly in the First World War (1914–18).

Origin

Early 20th century; earliest use found in Army & Navy Gazette. Probably arbitrary (or perhaps from the initial letter of query), probably after slightly earlier use of P-boat for a patrol boat.