cannot make head or tail of something
can't make heads or tails (out) of (someone or something)
Cannot understand someone or something at all; cannot make any sense of someone or something. I've been working with the new software for hours, but I still can't make heads or tails out of it. I can't make heads or tails of that new guy in accounting. Sometimes, he's really friendly, and then other times he acts like he's never met me before.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
cannot make head or tail of somethingor
cannot make head nor tail of somethingmainly BRITISH
If you cannot make head or tail of something or cannot make head nor tail of it, you cannot understand it at all. I couldn't make head or tail of the instructions. They couldn't make head nor tail of the new computer system.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012