when a thumb is inserted into the anus during sexual activity

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.

one or more thumbs in the ass

Came from the indian term for "thumbing" a fellow student