A black butterfly is a butterfly often symbolizing death. Though it does not mean "to die" but to have something come to it's end. These are said to appear to foretell the end of something, and said to appear to one who is near death. They are both mythical and real. I mean mythical because of the stories accompanied with them, and they are real because there are many butterflies who are many black.

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.

Pussy that got fucked so much it looks like a floppy old black butterfly.