* Physical Grinding: The Green Knight's axe is described as being sharp, almost to the point of being "grinding." This literal, physical grinding is a metaphor for the Green Knight's verbal attacks and challenges. He is sharpening his words, just as he sharpens his axe, to inflict pain and test Gawain's mettle.
* Figurative "Word Whetting": The phrase "word whetting" refers to the act of sharpening one's wit or language. The Green Knight is a master of this. He uses riddles, double entendres, and provocative language to challenge and unsettle Gawain. This verbal sparring is a form of "word whetting," as both characters try to outwit and outmaneuver the other with their words.
Therefore, the grinding noise of the axe, the literal act of whetting, is a metaphor for the verbal sparring and the metaphorical "word whetting" that takes place between Gawain and the Green Knight throughout the poem. The grinding noise acts as a constant reminder of the impending danger and the challenging nature of their encounter.