back of one's hand - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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).