Hellacious \he-LEY-shuhs\ , adjective;
1. Remarkable; astonishing
2. Formidably difficult
This US slang word is first attested in 1847 and is probably modeled after the slightly older bodacious. Bodacious, despite its slang popularity in the 1980's, was actually first attested in 1845 with the definition "complete, thorough, arrant."