1
general::
Also, drop a clanger. Say something indiscreet, commit a social gaffe. For example, John dropped a brick when he called her by his ex-wife's name. [Slang; 1920s]
American Heritage Idioms
2
general::
Phrase(s): drop a brick
Go to drop a bomb(shell) and explode a bombshell; drop a brick
McGrawhill's American Idioms And Phrasal Verbs