The episode: “Revenge of the Creature,” ep. 801
The riff: Sullenly intoned by Tom as the captured Creature from the Black Lagoon morosely kneels in his underwater enclosure.
The explanation: It’s a quote from “Hamlet”–more accurately, the beginning of Hamlet’s first soliloquy in Act 1, Scene 2. At this point he is distraught over his father’s death and mother’s re-marriage, and laments that suicide is frowned upon by the higher power. It’s just typical MST3k writing–applying Shakespearean quotations to black and white movie monsters.
Novelty factor: I don’t know Shakespeare nearly well enough to actually recognize specific quotations (although a few readers of this blog will), but when you hear the way Tom speaks the line, it’s pretty clear he’s probably quoting The Bard from the context.