牢固
láo gù · HSK 7
firm
🔒 牢 (pen, animal fold) + 固 (solid) = penned-up solid, so firm and secure
牢 originally meant a livestock pen built sturdy enough to hold animals; pair it with 固 (solid, fixed) and you get 牢固, firm and secure. Both halves carry the sense of something locked down and unmovable.
牢 — roof 宀 + ox 牛
Originally a pictograph showing an ox (牛) penned under a roof or enclosure (宀), depicting a livestock pen or stable. From 'animal pen' it extended to mean 'prison' (a place of confinement) and then 'firm, secure, sturdy' — the sense of something solidly enclosed.