Be all mouth
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
The phrase is casual and best suited for informal conversations, not formal settings. 
My brother always boasts about his cooking skills but when asked to cook, he's all mouth.
Pro Tip 2/3Negative Connotation
Using 'be all mouth' implies a negative opinion about someone's credibility or seriousness. 
She claims she'll start her diet every Monday, but she's all mouth.
Pro Tip 3/3Emotional Impact
The phrase can be offensive or hurtful if directed at someone; use with caution. 
Jake said he would help us move, but he was all mouth.