'decayed, musty substance'(Modern English rottack)
Formed within English with suffix -ock on either (1) the v. continuing OE gerotian 'to rot, putrefy' (similar wk. verbs derived on the zero grade of a PGmc st. class II v. *reutan- (on the evidence of the OIcel pp. rotinn 'rotten', see further roten) are found across the NGmc and WGmc languages, cp. OFris rotia, MDu rotten, OS rotōn, OHG rozzēn, OIcel rotna) or (2) rot (n.). MED and OED3 compare ME mollok(e) 'rubbish, debri' < mol(le) ' dust, ashes, dirt, trash'.
(1) *rut-; (2) *xrut- or *rut-
Proposed ON Etymon (OIcel representative)
Other Scandinavian Reflexes
(2) Far rot, Icel rot, Norw rot, Dan råd, Sw dial råt, rått
(2) hrot, rot 'scum, mucus'
Phonological and morphological markers
Hapax legomenon in ME; OED3 connects it with MnE dial rottack (see also EDD and Anderson 344n).
Occurrences in the Gersum Corpus