drown

From Middle English drownen, drounen, drunen (“to drown”), of obscure and uncertain origin.

The OED suggests an unattested Old English form *drūnian. Harper 2001 points to Old English druncnian, ġedruncnian (> Middle English drunknen, dronknen (“to drown”)), "probably influenced" by Old Norse drukkna (cf. Icelandic drukkna, Danish drukne (“to drown”)). Funk & Wagnall's has 'of uncertain origin'. It has been theorised (see e.g. ODS) that it may represent a direct loan of Old Norse drukkna, but this is described by the OED as being "on phonetic and other grounds [...] highly improbable", unless one considers the possibility of an unattested variant in Old Norse *drunkna.