"Tree deer?" That sounds like a reference to something awfully specific, if only Joker could remember what. Sure, there's his general state of being, with the wings, and the antlers, but he can't really think of anything off the top of his head that would fit that bill. Well, there's the time he got drunk and stuck in a tree, but he's pretty sure that Julien doesn't know about that. Probably.

Which means.... This has to be a reference to something else. That period of blankness from being stuck in a wall to laying in a medical tent bed. Something, he imagines, must have something to do with the blurry and indistinct visions and dreams he sometimes has when his mind spaces out enough.