know one's ass from a hole in the ground

• know the difference between one's ass and a hole in the ground

know one's ass from a hole in the ground (third-person singular simple present knows one's ass from a hole in the ground, present participle knowing one's ass from a hole in the ground, simple past knew one's ass from a hole in the ground, past participle known one's ass from a hole in the ground)

1. (idiomatic, vulgar) To have an adequate level of knowledge or skill; to understand what one is doing or talking about.
   • 2004, David Drake, The Reaches‎, →ISBN:

     "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."

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

     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."

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

     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.”

• Almost always used in negative constructions to describe someone's ignorance or stupidity, such as: He doesn't know his ass from a hole in the ground.

• find one's ass with both hands and a flashlight
• (archaic) 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

• See know one's stuff

• know someone from a hole in the ground