"I'm your huckleberry."
Therefore, "I'm Your Huckleberry" literally means "I'm your Hero."
In current adaptaions, in reference to the movie Tombstone, it means "I'm your man." as an affirmative response to a challenge.
Wretched slugs, don't any of you
Have the guts to play for blood?
I'm your huckleberry.
Ringo turns. Doc stands there, smiling that Cheshire cat
That's just my game.
Taken form the offical script from the movie Tombstone.
"I'm your Huckleberry."
In the moveie he didn't say "I'm your huckleberry", he said "I'm your HUCKLEBEARER". His accent in the movie makes it hard to hear. In the 1800's little handles on a coffin were called "huckles", an English term. Instead of pallbearers the people who carried the coffin were called "hucklebearers" at the funeral.
This is why the other guy got so bent outta shape when Doc said "I'm your hucklebearer". He was telling the other guy "I'm your pallbearer" or literally I'm causing your funeral. This is why it was so offensive and the shooting started.
Doc: I'm your huckleberry.
Holliday: I'm your huckleberry
Jimmy: Well come on then pus.
Holliday: Pulls out 12 gauge shottie
-Boooooom- Jimmy dies
And I will jump into your game Like 'Huck Finn', and give you a "shel·lack·ing" (As the most Hon. Sir Sean Connery could only say properly). And we will "tear-this moTHa-out". +))
And you reply: "...I'm your huckleberry..."