1) A mark of the London cutlers.
From the early seventeenth century Sheffield cutlers were passing off their wares as London products by stamping them with the mark of a dagger, and in 1624 the London Cutlery Company seized from a Sheffield factor sixt dozen and sixe knyves which had the Dagger counterfeited upon them, being Sheffeild knyves.
2) The upright piece of wood nailed to the bars in the middle of a hurdle or gate.
Its function is explained in detail by Henry Best in a description of how folde barres were made: 1642 The third thinge belonginge to a barre is a dagger, which goeth straight downe the middle of the spelles and is nayled to each spell with a single 8 or 10 pennie nayle, Elmswell.