big of one
Generous of one, as in It was big of Bill to give his brother his entire paycheck. This expression may be used either straightforwardly (as above) or sarcastically, as in How big of you to save the absolute worst seat for me. [c. 1940]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.