give (someone) the shirt off (one's) back
To act in a very generous and selfless manner. I'm not surprised that Tammy loaned you money—she would give anyone the shirt off her back.
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.
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.”