solida

A false seam, and the band gives you the contrast for free. One page back sits kokido, which really is koko plus the offspring suffix -id-. Solida is not sol- plus -id-. There is no king-sized family of only-children hiding in it. The root is solid-, borrowed whole from Latin, and the -id- you can see is the Latin adjective ending -idus that came along inside it.

Name the class and it stops catching you. Esperanto has a good number of adjectives that end in -ida for exactly this reason: acida sour, rigida stiff, avida greedy, timida shy, humida damp, valida valid, stupida stupid. The test is mechanical. The real suffix -id- attaches to a noun root and produces a noun about a living creature's young: bovido, reĝido, kokido. These are adjectives, they end in -a, and stripping the -id- leaves nothing that means anything. Sol- would give you sola, alone, which has no possible route to firmness.

Three senses, and English blurs them the same way. The first is physical state: a solid rather than a liquid or a gas, which is why solidiĝi is to solidify and solido is a solid body. The second is build quality, a thing that does not give when you lean on it. The third is figurative and the most useful: a solida argumento is a sound argument, solida amiko a dependable friend.

The fence with the neighbours. Firma is about holding steady and not shifting, which is why a firm grip is firma and not solida. Fortika is about withstanding rough treatment, robust in the practical sense. Forta is strength as a capacity to act. Solida sits underneath all three and describes what the thing is made of rather than what it can do.