An injunction to put something out of one's hopes, concern, etc, esp because it is impossible: If you thought you were next in line around here, forget it/ Forget it, she never did intend to go/ I can get up there most of the time, but in winter, forget it
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 forget it
Overlook it, it's not important; you're quite mistaken. This colloquial imperative is used in a variety of ways. For example, in Thanks so much for helping—Forget it, it was nothing, it is a substitute for “don't mention it” or you're welcome; in Stop counting the change—forget it! it means “stop doing something unimportant” in You think assembling this swingset was easy—forget it! it means “it was not at all easy”; and in Forget it—you'll never understand this theorem it means that the possibility of your understanding it is hopeless.
[ c. 1900