Dirk
mid 16th century: of unknown origin.
wiktionary
Etymology unknown, apparently from Scots drik. First attested in 1602 as dork, in the later 17th century as durk. The spelling dirk is due to Johnson's Dictionary of 1755.
Early quotations as well as Johnson 1755 suggest that the word is of Scottish Gaelic origin, but no such Gaelic word is known. The Gaelic name for the weapon is biodag. Gaelic duirc is merely an 18th-century adoption of the English word.
A possible derivation is from the Scandinavian personal name Dirk (short for Diederik), which is used of lock-picking tools (but not of knives or daggers). Another possibility is that dork originates as a sailor's or soldier's corruption of dolk, the Dutch and Scandinavian form of German Dolch(“dagger”).
The American slang term may be a variant of dick(“penis”).
etymonline
dirk (n.)
"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.
