beat the meat - 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.
Masturbate, as in He was always beating the meat. This term, considered vulgar slang, is used for males, since it uses meat in the sense of “penis,” a usage dating from the late 1500s.