Stern
Old English styrne, probably from the West Germanic base of the verb stare.
wiktionary
From Middle English stern, sterne, sturne, from Old English styrne(“stern, grave, strict, austere, hard, severe, cruel”), from Proto-Germanic *sturnijaz(“angry, astonished, shocked”), from Proto-Indo-European *ster-(“rigid, stiff”). Cognate with Scots stern(“bold, courageous, fierce, resolute”), Old High German stornēn(“to be astonished”), Dutch stuurs(“glum, austere”), Swedish stursk(“insolent”).
Most likely from Old Norse stjórn(“control, steering”), related to stýra(“to steer”), from Proto-Germanic *stiurijaną, whence also English steer. Also possibly from Old Frisian stiarne(“rudder”), from the same Germanic root.
From a variant of tern.
etymonline
stern (adj.)
Old English styrne "severe, strict, grave, hard, cruel," from Proto-Germanic *sternjaz (source also of Middle High German sterre, German starr "stiff," störrig "obstinate;" Gothic andstaurran "to be stiff;" Old Norse stara; Old English starian "to look or gaze upon"), from PIE root *ster- (1) "stiff." Related: Sternly; sternness.
stern (n.)
early 13c., "hind part of a ship; steering gear of a ship," probably from a Scandinavian source, such as Old Norse stjorn "a steering," related to or derived from styra "to guide" (see steer (v.)). Or the word may come from Old Frisian stiarne "rudder," which also is related to steer (v.). Stern-wheeler as a type of steam-boat is from 1855, American English.
