be all mouth and no trousers

US /bi ɔl maʊθ ænd noʊ ˈtraʊzərz/
UK /bi ɔl maʊθ ænd noʊ ˈtraʊzərz/
"be all mouth and no trousers" picture
1.

口先だけで行動が伴わない, 有言不実行

to talk a lot about what you are going to do but fail to actually do it

:
He's always boasting about his plans, but he's all mouth and no trousers.
彼はいつも計画について自慢しているが、口先だけで行動が伴わない
Don't listen to her promises; she's all mouth and no trousers.
彼女の約束を聞くな。彼女は口先だけで行動が伴わない