Rejection or contempt, as in
Unimpressed with him, she gave the back of her hand to his suggestion. This phrase is usually the object of a verb such as
give or
show. [Second half of 1700s]
Back of the hand similarly means "an insult" in the term
back-handed compliment (see under
left-handed compliment) but has a quite different meaning in
know like the back of one's hand (see under
know like a book).