give the shirt off one's back, to

give someone the shirt off one's back

Fig. to give anything that is asked for, no matter the sacrifice required. Tom would give any of his old army buddies the shirt off his back. You can always count on Mark when you're in trouble; he'd give you the shirt off his back.
See also: back, give, off, shirt
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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.
See also: back, give, off, shirt
The American Heritage® Dictionary of Idioms by Christine Ammer.

give the shirt off one's back, to

To give one’s all; the ultimate in generosity. This hyperbole appeared in Tobias Smollett’s novel of 1771, Humphry Clinker, and has been employed ever since. Elinor Wylie used it in her ironic poem “Portrait in Black Paint”: “She’d give the shirt from off her back except that she doesn’t wear a shirt.”
See also: give, off, shirt
The Dictionary of Clichés by Christine Ammer
See also:
  • give anything (for something)
  • give (one) the works
  • give (one) static
  • don't give (something) a second thought
  • (Can I) give you a lift?
  • all in, be
  • be all in
  • give (one's) ground
  • give ground
  • give it a burl