[third sense from wide-open throttle, probably fr railroading]
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 wide open
Unresolved, unsettled, as in The fate of that former colony is still wide open.
Unprotected or vulnerable, as in That remark about immigrants left him wide open to hostile criticism. This expression originated in boxing, where it signifies being off one's guard and open to an opponent's punches. It began to be used more broadly about 1940. Also see leave open