A middle-aged man who somehow transformed into his younger self was smoking out of habit and he wondered
今俺がこの姿で吸い続けて元に戻った時、その辺どうなのかなって、突然肺がんとかカンベンなんだけど
I don't understand what カンベン means in this context. Its usual meaning of "forgive" does not make sense.
The base meaning of 勘弁 is indeed "to forgive". One of the most common usage patterns is 勘弁してください, which expresses a polite yet strong refusal, like begging for mercy ("Oh please don't ～", "I beg you, no ～").
内視鏡は勘弁してください。
I really hope I can avoid the endoscopy.
By further extension, 勘弁だ can express a simple strong refusal. It often implies that the situation is seen as an unjustified punishment or treatment.
内視鏡は勘弁だ。
Endoscopy is a definite no-go.
