The story
Uncertain, but dates to the late nineteenth century in the United States. Compare huckleberry in the sense "person of little consequence", or the idiom huckleberry above a persimmon.
Etymology from Wiktionary, CC BY-SA 4.0.
I'm your huckleberry
Today it means: I am your partner; I will join you; I will work with you; I will fight you; I will dance with you. Full definition →