Etymology

Via Proto-Italic *-isemos, from Proto-Indo-European *-is- (zero-grade of *-yōs ‎(“intensive/comparative suffix”)) + *-(t)m̥mo- ‎(“absolutive suffix”). The latter is seen whole in -timus (e.g. intimus – extimus, citimus – ultimus, dissimulated pessimus – optimus). The original form seems, however, to be *-m̥mo- (cf. super – summus, Gothic 𐌹𐌽𐌽𐌿𐌼𐌰 ‎(innuma, “innermost”)), with acquired t from the paradigm -ter- – -timus- (e.g. exter – extimus), and is cognate with Proto-Germanic *-umô (source, via metanalysis, of English -most). By various sound changes (e.g. *-isemos > *-ismos > -īmus) superlative grades in -īmus, -ēmus etc. (e.g. extrēmus, suprēmus) are also found.

The expected alteration of *-(o)ism̥mos to *-erimus is nowhere to be found. The -ss- may be an example of expressive lengthening of the *-s- or influence from specific forms as pessimus and endings in -errimus and -illimus.

Pronunciation

• (Classical) IPA^(key): /ˈis.si.mus/, [ˈɪs.sɪ.mʊs]
• (Ecclesiastical) IPA^(key): /ˈis.si.mus/