also: カッパのへ
expression, noun
something very easy; a cinch; a piece of cake
Idiomatic expression literally meaning 'a kappa's fart', used to describe a task that is trivially easy. Often used in the phrase 河童の屁とも思わない (to think nothing of).
See also: 河童
こんな仕事は河童の屁だよ。
This kind of work is a piece of cake.
彼は難しい試験も河童の屁とも
He seems to think nothing of even difficult exams.
From the folkloric creature kappa (河童) and 屁 (fart). The phrase plays on the idea that a kappa's fart in water would be insignificant and effortless, hence something trivially easy.