When she criticized his cooking, he instantly rejoined that at least his dishes wouldn't give anyone food poisoning.

rejoined 是动词 rejoin 的过去式形式。rejoin 由前缀 re-（表示"再次"）和 join（加入、参与）组成，原意是"重新加入"，引申为"回应、反驳"的意思。在句中表示"立即回应并反驳"的机智应答。