know one's ass from a hole in the ground

present singular third person knows one's ass from a hole in the ground · participle present knowing one's ass from a hole in the ground · past knew one's ass from a hole in the ground · participle past known one's ass from a hole in the ground · alternative know the difference between one's ass and a hole in the ground

1. idiomaticvulgarTo have an adequate level of knowledge or skill; to understand what one is doing or talking about.

   Foxx gave a comic's pause. "Just goes to show you, don't it? Some folks don't know their ass from a hole in the ground."

   2005 June 12, Gary M. Pomerantz, “First Chapter: Wilt, 1962”, in New York Times, retrieved 26 Jul 2014

   2 more examples

   "Why are you so sure and they aren't?" the landsman said. . . . "Because Mr. Ricimer knows his ass from a hole in the ground, sir."

   2004, David Drake, The Reaches, →ISBN

   In the unofficial ad, the facilitator says about the man whose wife will be replaced, “This guy wouldn’t know his ass from a hole in the ground.”

   2006 December 5, David Kiley, Chrysler "Whoops" Ads Part of Important New Genre, Bloomberg, retrieved 23 Sep 2016

   Synonyms: find one's ass with both hands and a flashlight, know B from a battledore, know one's ass from one's elbow, know one's head from a hole in the ground, know one's head from one's ass, know shit from Shinola, know what one is about, know which end is up