know one's ass from a hole in the ground

verb1

•
(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

forms

knew one's ass from a hole in the ground · known one's ass from a hole in the ground · knowing one's ass from a hole in the ground · knows one's ass from a hole in the ground