The story
From euphoria + -ous. According to The Century Dictionary, erroneously for *euphorous, from Ancient Greek εὔφορος (eúphoros) (whence euphoria via εὐφορίᾱ (euphoríā) – εὔφορος (eúphoros) + -ῐ́ᾱ (-ĭ́ā)).
Etymology from Wiktionary, CC BY-SA 4.0.