feroca

The seam that is not there, and it is a good one. Feroca looks like it ought to be fero, iron, plus some suffix, and English speakers half-feel it too: iron-hard, iron will, ferocious. It is not. Iron is not inside ferocity, and the reason is that Latin had two different fer- roots and Esperanto inherited both.

• Ferrum, with two r's, is the metal. Esperanto fero, English ferrous, ferric, farrier. The chemical symbol Fe is this one.
• Ferus, with one, means wild, untamed, of the woods. From it Latin built ferox, of a wild temper, and it also gives English feral and, by a French detour, fierce.

Feroca is the second family. It is one root in Esperanto, feroc-, and there is no oc suffix in the language for it to be the second half of. Beside fero it is a pure coincidence of spelling, and the sort of coincidence worth saying out loud because otherwise it quietly sits there feeling like a derivation.

What it actually describes. Wild cruelty in a fight or a quarrel: an animal that will go for you, a soldier who gives no quarter, a look that means it. By extension it does unbearable intensity in something unpleasant, so feroca malvarmego is savage cold and feroca doloro is ferocious pain. Feroco is ferocity itself, and ferocaĵo an act of it.

A register note that says something about the language. ReVo suggests kruelega or sovaĝe kruela in preference to feroca, and that preference is a small argument about what Esperanto is for. The language can build the sense out of pieces you already have: kruela plus the augmentative -eg-, or sovaĝa doing the work adverbially. A borrowed root that adds nothing the affixes could not is exactly what the purists object to. You will still meet feroca constantly in translated literature, where it renders a Romance original word for word, so know it - but if you are writing your own Esperanto, kruelega costs you nothing and one fewer root.