know (one's) ABCs
1. Literally, to know the letters of the alphabet. Wow, she's only two and a half, and she already knows her ABCs? That's very impressive! My grandfather never knew his ABCs, but he was still successful in life.
2. To know the essential or basic facts (about something). In this usage, "the" is sometimes used instead of "one's." I know you're new to a lot of this, so just stick with Sarah here until you know your ABCs. Look, I know this stuff isn't terribly exciting, but you need to know the ABCs of chemistry if you're going to become a doctor.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
know one's ABCs
Fig. to know the alphabet; to know the most basic things (about something). Bill can't do it. He doesn't even know his ABCs. You can't expect to write a letter when you don't even know your ABCs.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.