the grumblings or bitter remarks of someone who has lost or is defeated; sour-grapes complaining by a loser.
そんなことを言うのは負け犬の遠吠えだ。
Saying that is just sour-grapes complaining.
試合に負けた彼の発言は、聞いていて負け犬の遠吠えのようだった。
His remarks after losing the match sounded like the grumblings of a sore loser.
他人の成功を嘲るのは、結局は負け犬の遠吠えに過ぎない。
Mocking others' success is ultimately just a loser's whine.