Hello Khondoker! I agree with Duncan but I would also like to add that, as well as being a jokey response, it could also be a sarcastic way of saying "I would never do that" or "I could never do that" or "I couldn't even imagine doing that" or "how could you even think that I would do that". It is a way of saying that there is no possibility of such a thing ever happening. Here is an example: "My teacher told me not to slack off and neglect my schoolwork, as if I would....". I hope that helps!

It basically means 'I would not' but as mentioned before it is said in a tone that is either joking or sometimes in an angry way.

In my experience, this is usually a slightly jokey response. A bit "tongue-in-cheek". A: Did you tell them that my jokes are terrible? B: As if I (ever) would. Here the response implies that B thinks the jokes are bad and that A knows that. B is jokingly saying that they probably did say that, but they are being coy about it, perhaps a little bit sarcastic. B knows that it won't make A mad to know they said this. The joke, or humour, is that although B is saying they would never say that, they both know that B did (or probably did) say it.