Banging your girlfriend doggy style in her arse while reaching into her pussy (whole handed) jacking yourself off through the thin membrain that seperates the two.

A phrase to describe someone who is cognitively degenerating. Synonym of "going off the deep end". Can have varying degrees of severity.

Reference to the song "Hey You" by Pink Floyd. The line "and the worms ate into his brain" makes no sense in an otherwise linear and literal narration throughout the lyrics.