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The OED says:
3. A person, spec. (derog.) a person of little consequence.[1835 Gent’s. Vade-Mecum (Philadelphia) 22 Aug. 2/4 Orson, the wild man of the woods is nothing to himnot a circumstancenot a huckleberry.] 1868 New Eng. Base Ballist 3 Sept. 17/1 Now then, my huckleberry, look sharp! you’re wrong! 1889 ‘MARK TWAIN’ Connecticut Yankee 338 The Saracen..is no huckleberry.
Perhaps you should submit H.D. Thoreau’s passages to OED. They update their sources periodically.
Michael Quinion has a good response:
I’m your huckleberry.
What it means is easy enough. To be one’s huckleberry — usually as the phrase I’m your huckleberry — is to be just the right person for a given job, or a willing executor of some commission. Where it comes from needs a bit more explaining.
First a bit of botanical history. …