-Est

I. adjective suffix or adverb suffix Etymology: Middle English, from Old English -st, -est, -ost; akin to Old High German -isto (adjective superlative suffix), Greek -istos — used to form the superlative degree of adjectives and adverbs of one syllable
Example: fattest
Example: latest, of some adjectives and adverbs of two syllables
Example: luckiest
Example: oftenest, and less often of longer ones
Example: beggarliest II. verb suffix or -st Etymology: Middle English, from Old English -est, -ast, -st; akin to Old High German -ist, -ōst, -ēst, 2d singular ending — used to form the archaic second person singular of English verbs (with thou)
Example: didst
Example: canst

Merriam-Webster's Collegiate Dictionary