Can't make head nor tail (out) of something - someone

Can't make head nor tail (out) of something / someone

Meaning:

Not be able to understand

If you can't make head nor tail (out) of something or someone, you can't understand it/him/her at all.

Example:

Could you show me how to use this machine? I can't make head nor tail out of the instructions in the manual.

Bob is a very strange guy. Everything he says and does is very strange. I cannot make head nor tail of him.