-est 1
a suffix forming the superlative degree of adjectives and adverbs: warmest; fastest; soonest.
[ ME; OE -est, -ost. Compare Gk -isto- ]
-est 2
a native English suffix formerly used to form the second person singular indicative of verbs: knowest; sayest; goest.
Also, -st .
[ ME; OE -est, -ast, -st, 2nd pers. sing. pres. indic. endings of some verbs ( -s earlier verbal ending + -t, by assimilation from thu THOU 1 ) and 2nd pers. sing. past endings of weak verbs (earlier -es + -t ) ]