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.

JH 2011
Knife Blade

Source: Wikimedia Commons.