Source : Webster's Revised Unabridged Dictionary (1913)

-est \-est\ [AS. -ost, -est; akin to G. -est, -ist, Icel. -astr,
   -str, Goth. -ists, -?sts, Skr. -ish?ha.]
   A suffix used to form the superlative of adjectives and
   adverbs; as, smoothest; earl(y)iest.