give the shirt off one's back
Give anything and everything one possesses. For example, Tom is truly generous—he'll give you the shirt off his back. This hyperbolic idiom was first recorded in 1771.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.