What is the origin of 'sleep tight'?
Read moreOne of the mysteries of the English language finally explained.
Stubbornly refusing to change one's opinion or course of action.
Late Middle English (originally in the sense ‘hardened in sin, impenitent’): from Latin obduratus, past participle of obdurare, from ob- ‘in opposition’ + durare ‘harden’ (from durus ‘hard’).