big deal
/ˌbɪɡ ˈdiːl/originally US
noun
1An important business transaction or contract.
2Something considered important; a cause for excitement or concern.
3Used as an ironical exclamation to express one's contempt for something regarded as impressive or important by another person.
Origin
Early 20th century; earliest use found in Frank Norris (1870–1902), novelist. From big + deal.