"stabbing weapon, dagger, poinard," c. 1600, perhaps from Dirk, the proper name, which was used in Scandinavian for "a picklock." But the earliest spellings were dork, durk (Johnson, 1755, seems to be responsible for the modern spelling). The earliest association is with Highlanders, however there seems to be no such word in Gaelic, where the proper word is biodag. Another candidate is German dolch "dagger."
The masc. given name is a variant of Derrick, ultimately from the Germanic compound in Dietrich.