Hi Hyppolite - you should come and join us in the English only forum for questions like this - you'll get a larger range of answers there I think, as opposed to here which is more about the best way to translate from English to-from French.
No, no. I can answer... there is the expresion on AOL called "roflmao" which is roll on floor laugh my ass off, so To laugh off your ass, is a slangish-type to laugh extremley hard. the part, "like a tea-kettle" would be like how you pour out teaor off. I have lived with this language and learned many negative expressions, but this is not one of them, so if you still dont understand, it would not kill you not to know.
Guess not. It was just something I've had on my mind for quite some time now. I thought I'd ask. Thanks anyway Mikey.
PS : I thought, maybe, there would be a historical explanation, like for the french expression "tirer au flanc" which doesn't have much of a meaning either until you get that explanation.