knife / nīf/ • n. (pl. knives / nīvz/ ) a cutting instrument composed of a blade and a handle into which it is fixed, either rigidly or with a joint. ∎  an instrument such as this used as a weapon. ∎  a cutting blade forming part of a machine.• v. [tr.] stab (someone) with a knife. ∎  [intr.] cut like a knife: a shard of steel knifed through the mainsail.DERIVATIVES: knife·like / -ˌlīk/ adj.knif·er n.

knife a knife is the emblem of St Bartholomew, St Peter the Martyr, a 13th-century Dominican friar and priest born in Verona, who was attacked and killed while travelling from Como to Milan (he was wounded in the head with an axe, while the friar who was with him was stabbed), and St William of Norwich.
before you can say knife very quickly; almost instantaneously.
like a hot knife through butter very easily; without any resistance or difficulty.
twist the knife in the wound deliberately make someone's sufferings worse.

knife sb. XI. Late OE. cnīf — ON. knīfr = OFris., MLG. knīf, MDu. cnijf (Du. knijf) :- Gmc. *knīƀaz, of uncert. orig.
Hence vb. XIX.