noun
proper method; way to do something; means
Used almost exclusively in negative or desperate contexts, such as せんすべがない or せんすべなく, meaning 'there is no way to deal with it' or 'at a loss for what to do'. The word is somewhat literary and old-fashioned.
突然の事故に、為ん術もなく立ち尽くした。
Faced with the sudden accident, I stood there helplessly with no means to act.
彼はせんすべなく、ただ空を見上げていた。
He had no recourse and just looked up at the sky.
なす術 is a more common variant with the same meaning, often used in the set phrase なす術もない. せんすべ is rarer and more literary.
The derivation is uncertain. The kanji 為ん is a rare ateji for せん, and 術 means 'method'. The word likely originated as a set phrase meaning 'the way to do it'.