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