1. not able to understand or make sense of something/someone
I can't make head nor tail/heads or tails of the document
To not be able to make head or tails of something (not be able to make head nor tail of something) means not to be able to understand something in any way. If you can't make head or tails of something, it means you can't understand the problem or how to approach it. This is a popular idiom used in social and professional contexts.