noun
spiteful person; sore loser
Refers to someone who is easily frustrated or resentful when things don't go their way, often used for children or in a mildly teasing manner.
弟は負けるとすぐ泣く悔しん坊だ。
My little brother is a sore loser who cries as soon as he loses.
あの子は悔しん坊で、ゲームに負けると拗ねてしまう。
That kid is a spiteful one; he sulks whenever he loses a game.