【Talk a big game】
-Justin talks a big game about standing up to the police, but runs like a coward at first sight.
-Daenarys talks a big game about going to war with the neighbors, even though she will probably loose.
-The boxer talked a big game for the upcoming match, but got knocked out.
Definition: They are talking about how they can defeat someone, but they can't back this claim up.
Etymology: Literally talking big.
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Other examples:
1. He lost everybody's trust by talking a big game.
2. He talked a big game when the boss was looking for someone to handle the problem but screwed up the whole thing.