From Classical Japanese せん (sen), from はせん (wa sen), with は merged with the preceding syllable or becoming や, giving やへん (yahen). In turn, はせん is a composition of は (wa, topic marker or emphasis marker), せ (se, “to do”, irrealis stem) and ん (n, “not”).
- (Kansai) negates verbs in non-past tense
- This word is morphologically an inflectional suffix. It is classified as 助動詞 (jodōshi, “auxiliary verb”) in traditional Japanese grammar.