the reason a butterknife is not very sharp is so you can lick them clean without cutting your tongue
i had to reread this thrice before realisimg you mean an actual knife that you smear butter with.
before that i was wondering about how this relates to the spy from team fortress 2, lol
the spy uses a butterfly knife
did you misread and/or do you not call the kind of knife you use to spread things on bread and, along with a fork, as common cutlery a butterknife when needing to specify from other kinds of knife?
i was aware that its called a butterknife but i rarely refer to it in english. the german name translates to "bread knife" so my brain made the connection to TF2, because i hear the term a lot more there.
a "butterknife" is a somewhat mocking term used for when a spy player kills their target using their knife, but without backstabbing them. its kinda synonymous with the term "facestab"