get a fix on

[Tdol]

Please calm down a bit here; there's no need for arguing.

In answer to the original question, it is difficult to say exactly what it means without further context, but my guess would be that the person means that it is hard to understand it completely, though I have no idea whether this is in a medical sense or a philosophical one.

[nimsooze]

"His colleague Jean-Paul Sartre has a somewhat less upbeat narrative to tell of the body as that "outside" of ourselves which we can never quite get a fix on , that unmasterable otherness which threatens to deliver us to the petrifying gaze of the observer."

[Tdol]

The clue is 'umasterable'- the body exists outside our personality and we cannot master or fully comprehend it. (That's my interpretation)