To degenerate cognitively, to be in the process of having a mental breakdown, the process of going crazy
Jeff: I can't believe what my pops says sometimes. It's like he's incapable of higher thought!
Julian: Yeah, I'd say your pops is going off the deep end LOL!
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Guy 1: *deep voice impression*
Guy 3: I didn't know it could go that deep...
Guy 2: That’s what she said.