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 <fattest> <latest>, of some adjectives and adverbs of two syllables <luckiest> <oftenest>, and less often of longer ones <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) <didst> <canst>
New Collegiate Dictionary. 2001.