the.com/stiff upper lip

means The quality of staying outwardly composed and uncomplaining even when you're hurting, scared, or grieving inside.

from The phrase is actually American, not British, first appearing in early-1800s U.S. writing — a trembling upper lip being an obvious, visible sign of someone about to cry or lose composure, so keeping it 'stiff' meant holding steady. It only became stereotyped as quintessentially British later, as shorthand for the reserved, emotion-suppressing national temperament the culture prized (or mocked) — a classic case of an idiom's popular association drifting far from its actual birthplace.