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

Verb

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.
   • "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

   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