impudent talk; talking back; getting the last word
Refers to stubbornly talking back or making needless retorts, often when one should stop. Carries a negative nuance of being argumentative or cheeky.
He talks back even when he's being scolded.
Before you make a needless retort, admit your own mistake.
From 減る (to decrease) in its negative form 減らず (not decreasing) + 口 (mouth), literally 'a mouth that doesn't decrease', i.e., someone who won't stop talking back.