it's no joke; gimme a break!
Used to express strong disbelief, annoyance, or refusal, often when something is unreasonable or absurd. Can be a literal statement that something is serious, or an exclamation meaning 'you must be kidding!'
You want me to do all this work alone? Gimme a break!
It's no joke that nobody was hurt in that accident; it's a miracle.
Literally 'it is not a joke', from 冗談 (joke) + じゃない (is not). The phrase is a fixed expression that has become idiomatic for expressing disbelief or refusal.