letter bomb

WordReference Random House Unabridged Dictionary of American English © 2026

let′ter bomb′, 

1. an envelope containing an explosive device designed to detonate when the envelope is opened by the recipient.

Also called mail bomb. 

• 1945–50

Collins Concise English Dictionary © HarperCollins Publishers::

letter bomb noun

1. a thin explosive device inside an envelope, detonated when the envelope is opened