(E=Englishman; G=girlfriend)
G: Have you gone mad? Why are you roaring your head off like a lion?
E: Rrrrr...ah!
G: Hey, have you gone off your rocket or what? Cut that out!
E: I'm letting off steam. The guy on TV said it's good for me.
G: Oh, really? Meowww! Meowww!
E: Hey, what are you doing?
G: The girl on the radio said meowing like a cat helps to cure indigestion.
E: Come on, that's absurd. You can't believe everything people say, you know.
G: Look who's talking! 
E: Uh...OK, wise guy. You win.

let off = release
That busy businessman let off steam by playing tennis three times a week. 散发多余精力
be mad at 对...愤怒      be mad about 对...疯狂
go off one's rocket = go crazy
cut it out = stop it
spirits
let up = stop
groceries  杂货
doubtful
on the radio  不说：in the radio(在内部，里面)
on TV  不说：on the TV
The pot is calling the caddle. 半斤八两
for good = forever  adv.