Ultimately from Proto-Indo-European *-mHo-, cognate to Proto-Germanic *-umô (whence English -most), via Italo-Celtic *-ism̥mo-, *-isemo-. Various sound changes and contractions led to various forms, including -imus, -īmus, -ēmus, as evidenced by irregular superlatives.
- added to adjectives to form superlative
Typically added to the adjective's genitive stem. Adjectives that end in -er instead take the suffix -rimus (e.g. liber > liberrimus). Some adjectives have irregular superlatives, e.g., bonus, optimus; malus, pessimus; magnus, maximus.