The word "foin" caused some trouble in this week's Saviolo class. Fortunately, the OED is to the rescue: "To make a thrust with a pointed weapon..." It occurs as both transitive and intransitive verbs and also a noun.
The etymology is uncertain, but best guess is from an Old French word for three-pronged fish-spear (which I would call a "trident").
It seems that in the 17th C the word merged with "foil" (a blunted sword).