Gravel dick: This is an imaginary sexual act motivated by extreme hatred. The perpetrator develops an erection, then slathers their penis in crazy glue, followed by the application of copious amounts of aquarium gravel. The perpetrator then sodomizes the victim repeatedly causing torrential anal bleeding, and permanent anal incontinence. As this rather despicable act causes rather intense pain, and perhaps permanent injury to the perpetrator (as well as the victim) it portrays the depth of hatred towards the victim (i.e., I hate you so much I would do gravel dick on you). You have to really hate someone to do gravel dick on them. Most often used as a mental revenge fantasy against those in the work place. Somewhat comparable to donkey punch in that it is imaginary, and involves violent inappropriate sexual content.
Mark was so angry at his boss for blaming him for another's actions that he daydreamed about gravel dicking him for the rest of the afternoon.