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, from Proto-Germanic *stiurijaną, whence also English steer. Also possibly from Old Frisian stiarne ("rudder"), from the same Germanic root.