fortis

Fortis, forte is a third declension adjective of two terminations: one form for masculine and feminine, another for neuter. It is an i-stem, so the ablative singular is forti, the genitive plural fortium and the neuter plural fortia. Comparative fortior, superlative fortissimus, adverb fortiter.

Both halves of the meaning are live. Of a wall or a body it is physically strong; of a person it is brave, and Roman writers slide between the two without warning. The trap is the adverb: it is fortiter. Forte, which looks like the obvious candidate, is the ablative of fors and means "by chance, as it happened" - a completely different word that turns up constantly in narrative. English fort, fortress, fortitude, comfort and effort all descend from fortis.