No kidding is said in surprise, or in flat disbelief. No kidding, he really did it.
It works both ways, so it can mean I am serious or I am not impressed. It is American and informal, and the tone decides which sense is meant.
1phraseno kiddingSaid in surprise, or in flat disbelief.
“He walked out on the spot, no kidding.”
no kidding is a phrase.
Practised in Small Talk & Everyday Phrases