From Middle English -est, from Old English -est, -ost, from Proto-West Germanic *-ist, *-ōst, from Proto-Germanic *-istaz, *-ōstaz, related to comparative -er. Cognate of Afrikaans -ste, Dutch -st, German -(e)st, Danish -(e)st, Swedish -(a)st.
Via PIE cognate with Ancient Greek -ιστος (-istos), see Pleistocene, protist.