Perhaps ultimately from the nominative masculine singular of oppositional adjectives in Proto-Indo-European *-teros; perhaps extended from the suffix in prepositions like inter, praeter.
-ter (comparative -tius, superlative -tissime)
- -ly; used to form adverbs from adjectives.
The suffix -ter is usually added to a third-declension adjective or participle stem to form an adverb of manner.
- prudenter (“intelligently, wisely”), from prudēns (“knowing, experienced”)
- dissimulanter (“dissemblingly, secretly”), from dissimulāns, present active participle of dissimulō (“dissemble, conceal”)