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.
See also: head, make, of, tail
Farlex Dictionary of Idioms.

cannot make head or tail of something

or

cannot make head nor tail of something

mainly 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.
See also: cannot, head, make, of, something, tail
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • can't make head nor tail of (someone or something)
  • can't make head or tail of
  • can't make head or/nor tail of something
  • can't make heads or tails (out) of (someone or something)
  • can't make heads or tails of
  • I can't even
  • can't even
  • cannot
  • cannot but
  • (Are) you kidding (me)?