Sentence:  Almost as an afterthought, we're given an origin story for Barney's alcoholism: he was once a sober, studious, Ivy League-bound high school scholar before Homer forced a beer on him that transformed him into a drooling, slurring, out of control rampaging <WSD>id</WSD>. 
Meaning of the word marked with <WSD> tags:  The unconscious impulsive component of the (personality) in the Freudian psychoanalytic model.