Kinch, the knifeblade
As Mulligan affirms soon after first calling Stephen "Kinch," he is the source of this nickname: “my name for you is the best: Kinch, the knife-blade.” Ellmann confirms that Gogarty gave the name to Joyce. He remarks that it imitated “the cutting sound of a knife” (131), but the association seems obscure.
If Kinch does mean knife, it coheres with an abundance of knives in the early pages of Ulysses: Mulligan’s razor; the knife on which, “impaled,” he thrusts slices of bread toward Stephen and Haines; the doctor's “lancet” and the writer's “cold steelpen." One effect of all these sharpened blades is to liken the Martello fortification to those at Elsinore, where hiding behind an arras or joining a fencing match can prove fatal, and Ithaca, where a host of suitors is slaughtered to the last man and the palace floors run ankle-deep in blood.