The original stiletto starts out in 15th century Italy as a thrusting knife, commonly lacking any edges for cutting. It is stout, narrow, and pointed for thrusting into the vitals of a victim. They were relatively easy to make considering they were square, triangular, diamond cross section, and even round. They are thought to have derived from the rondel daggers, but may have been inspired by the Roman stylus for inscribing clay tablets.
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It isn't until much later that people start to refer to folding, switchblade, and out the front narrow pointed folding knives as "stilettos".