“I have a weakness for black coffee ,which has your socks knocked off.”
’have a weakness for something’ means ’be unable to resist the temptation of something’ .So he said that he likes black coffee very much.
But "to knock your socks off" is a phrase meaning to amaze,impress or astonish you.
Is it appropriate to use this phrase in the conversation scene in the picture above? I don’t quite understand the meaning.