You And What Army? American slang
'What' can be replaced by 'whose' without chaging its meaning.
A sarcastic or ironic reply to a threat from someone whose ability to carry out the threat is not taken seriously.
What are you going to do, try to mess up my plan? You and what army?
A: We're gonna stop you right there. B: Oh yeah? You and what army?
Used to express that you have no sympathy for one's sadness.
To underestimate someone, something or oneself
To have low respect or regard for someone or something; To evaluate someone or something with a negative attitude
1. To consider something as true, real or available without questioning it
2. To fail to show proper appreciation or gratitude to someone or something; to underestimate the value of someone or something
The phrase is an American slang from at least the mid 1950. Its exact origins are unknown but it could be an exaggerated variant of the idiom 'you and who else'. The phrase is first recorded in Dale Plays the Game, a novel by Ann Forester:
“What are you two looking so fussed about? Who have you murdered and where have you hidden the body?”
“I’ll pull your ears for you, Slats!” Brant’s voice sounded natural once again.
“You and what army?” teased his sister.