Mulligan's nickname for Stephen, who he describes as a "knife-blade" (the OED defines a kinch as a type of knot and I've never been able to find the word defined as a knife, but for the purposes of the novel it makes the most sense to go with Mulligan's definition). This nickname, which alludes to the sharpness of Stephen's intellect, is used somewhat patronizingly–Mulligan recognizes that Stephen has greater intellectual powers than him and is passively-aggressively jealous, but is also aware that Stephen does not use his mind to as great a social advantage as Mulligan does.
