faint echo

Hugh Grant quipped, "Very big-mouthed! Literally, physically, she has a very big mouth. When I was kissing her I was aware of a faint echo."

what does 'faint echo' mean?

[Mister Micawber]

I presume that you found out what an 'echo' is-- the repetition of a sound when it bounces back from a surface, commonly encountered in large enclosed spaces like caves and tunnels. 'Faint echo' suggests that the enclosed space is very big indeed, as the reflected sound had to carry a long distance. Quite a big mouth.

i see!thx very much!