noun
easy money; ill-gotten gains
Refers to money obtained with little effort, often through luck or dishonest means. Carries a slightly negative or dismissive nuance.
ギャンブルで儲けた金はあぶく銭だから、すぐに使ってしまった。
The money I won gambling was easy money, so I spent it right away.
あぶく銭は身に付かないと言う。
They say easy money doesn't stick with you.
不労所得 is a neutral economic term for unearned income, while あぶく銭 is a colloquial, slightly negative term for easy or ill-gotten money.
From あぶく (bubble, foam) and 銭 (money), implying money that appears and disappears like bubbles. The exact historical derivation is uncertain.