... "Grendel means 'grinder,' a grinder of bones—that is, carnivorous."  This etymology of Ettmüller's may doubtless be correct,... (Journal of Education, v.9, 1877)

Grimm connected the name Grendel with O.E. grindel, OHG krintil, MHG grintel, "bolt, bar, riegel," and compared it with Loki, the name of the evil god in Norse Mythology, which he connected with ON lúka, "claudere."  Grendel would, according to this view of his name, be a being which "shuts in, incloses."  A similar use of Germ. riegel occurs in höllriegel, "devil."...
... Grendel is, of course, from an earlier form *grandil, not from *grendil, for a Prim. Germanic e before a nasal must become i, cf. OE. bindan : helpan.  *Grandil may without difficulty be derived from the Latin adjective grandis, "large, huge," and its use as the name of a huge monster may be due to the Latin source of the Grendel fable for in such a source this monster may have been most naturally described as grandis, "huge, monstrous."  The stem of the word is Latin, the ending has been Anglicised.  Grendel is, therefore, so far as form and meaning are concerned, a close parallel to strengel, "macht habend, herrscher, strong chief," Beowulf 3116, which is an expansion by the suffix -il(a) of the adjective strang, strong, "strong, mighty."  Just as strengel means "the strong one," so Grendel means "The Huge One, The Monsterous One." (Modern Language Notes, v. 19, 1904)