Middle English grotes, going back to Old English grotan, masculine weak plural, presumably a variant of the neuter noun grot "particle, whit" (also glossing Latin pollis "finely ground flour"), probably going back to Germanic *gruta-, noun derivative of *greutan- "to grind, crush" — more at grit entry 1
Note: Old English grotan is corrected from the manuscript form gratan, which occurs only once. Forms in other Germanic languages directly comparable to *gruta- appear to be lacking.