can't make head or/nor tail of something
can't make head nor tail 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 head nor tail of it. I can't make head nor tail of that new guy in accounting. Sometimes, he's really friendly, and then other times he acts like he's never met me before.
can't make head or tail of
Also can't make heads or tails of. Fail to understand, be quite confused about, as in I can't make head or tail of these directions. A version of this term dates back to Roman times, when Cicero wrote Ne caput nec pedes ("neither head nor feet") to describe confusion. In the current idiom the precise allusion is unclear: head and tail may mean top and bottom, beginning and end, or the two sides of a coin. [Second half of 1600s]