Loose cannon

Meaning

An unpredictable person who might cause damage.

Usage

Used to warn about unstable colleagues.

Origin & History

Origins and dates here are approximate summaries. Many idioms have disputed or unknown origins, and not every date has been checked against a historical dictionary, so treat specific dates and stories as a starting point rather than a citation.

Naval cannons without ropes that could fire unexpectedly.

Naval slang passing into general usage by the 20th century.