Whenever I ask a trans person to define gender I get some wishy-washy "well it's hard to define" "it's unique to everyone how they experience it" and so on. I've often gone into these conversations saying things along the lines of "I don't understand what gender is" or "I don't conceptualize my own gender" and often I'll hear in response something like "Gender is hard!" and "You might be agender, then!" which I think is their most rock-solid way to avoid criticism: redefine "this doesn't make sense" into it's own gender identity.