Old English sincan (intransitive) "become submerged, go under, subside" (past tense sanc, past participle suncen), from Proto-Germanic *senkwanan (cf. Old Saxon sinkan, Old Norse sökkva, Middle Dutch sinken, Dutch zinken, Old High German sinkan, German sinken, Gothic sigqan), from PIE root *sengw- "to sink."
The transitive use (mid-13c.) supplanted Middle English sench (cf. drink/drench) which died out 14c. Related: Sank; sunk; sinking. Sinking ...