Definition from Wiktionary, the free dictionary
- The basic idea of how to do something.
- I would help you, but I don't know the first thing about gardening.
- Used other than as an idiom: see first, thing.
1913, Joseph C. Lincoln, chapter 1, in Mr. Pratt's Patients:
- I stumbled along through the young pines and huckleberry bushes. Pretty soon I struck into a sort of path that, I cal'lated, might lead to the road I was hunting for. It twisted and turned, and, the first thing I knew, made a sudden bend around a bunch of bayberry scrub and opened out into a big clear space like a lawn.
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