make head or tail of something, to be unable to

To fail to understand something. This term was already used to denote total confusion by Cicero (Nec caput nec pedes, meaning “neither head nor feet”). Whether it refers to beginning or end, or top or bottom, or, as someone suggests, the two sides of a coin, is not known, but to make neither head nor tail of something has been so used ever since, in English from the seventeenth century to the present.