expression
easy mark; sitting duck; desirable prey
Idiomatic expression literally meaning 'good duck', used for a person who is an easy target for scams, exploitation, or manipulation. Often used in contexts of fraud, seduction, or taking advantage of someone naive.
彼は詐欺師にとってはいいカモだ。
He's an easy mark for con artists.
あの客はいいカモだから、高い商品を売りつけよう。
That customer is a sitting duck, so let's push the expensive products on him.
From いい (good) + カモ (duck). The 'duck' metaphor for an easy target comes from the idea that ducks are easy to hunt. The exact origin of the phrase is uncertain, but it is a common colloquialism.