Do as you please •Often an ironic acquiescence to something ill advised: You want to tell the cop he's wrong? Be my guest(1950s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with be my guest
Do as you wish. For example, May I drive your car?—Sure, be my guest, or Do you mind if I go to the play without you?—No, be my guest. This expression not only literally invites someone to behave as one's guest (using one's house, belongings, etc.) but also figuratively tells someone to feel free to act as he or she pleases.
[ ; c. 1950