"mender of kettles, pots, pans, etc.," late 14c. (mid-13c. as a surname), of uncertain origin. Some connect the word with the sound made by light hammering on metal. Tinker's damn "something slight and worthless" is from 1824, probably preserving tinkers' reputation for free and casual use of profanity; the plain and simple etymology is not good enough for some writers, and since 1877 an ingeniously elaborate but baseless derivation has been circulated claiming the second word is really dam.
1590s, "to work as a tinker," from tinker (n.). Meaning "work imperfectly, keep busy in a useless way," is first found 1650s. Related: Tinkered; tinkering.