rigida

Another -id- costume. Acida already made this case, and rigida is the next one along: -id- is a real and productive suffix meaning offspring (kato gives katido, reĝo gives reĝido), so the eye tries to cut a root rig- out of the front. There is no such root. Rigid- is one borrowed unit from Latin, exactly like rapida, solida and stupida. The suffix-hunting reflex is the right reflex in Esperanto - it is just wrong on words that arrived pre-assembled from Latin, and this band is full of them.

Rigida, malmola, firma - which one. Malmola is hard to the touch: a stone, a stale loaf. Firma is steady, not wobbling, and is usually approving. Rigida is stiff, unable to bend, and it is usually a small complaint: a rigid branch, a rigid collar, a rigid back after a long journey. A thing can be malmola and not rigida (a marble ball) or rigida and not especially malmola (a sheet of card).

The figurative use travels. Rigidaj reguloj are inflexible rules, rigida pensado is rigid thinking, and rigida vizaĝo is a face giving nothing away - all of them exactly as in English, which for once you can rely on. The opposite is fleksebla, bendable, built from fleksi (to bend) plus the -ebl- of possibility. Rigideco is the noun, rigidiĝi is to stiffen up.