What does im your huckleberry mean

[Delmonico]

Doe you use SAE 10W-40?

[River City John]

Naw, just sweet oil, as you know.
Shine 'em up pretty fer the customers.

[Tsalagidave]

'Huckleberry' was a well-known slang term from the period and the context in the film in which it was used means "I'm of a skill level you can handle." Kilmer's Holiday character first says it coaxingly as he saunters forth with a pistol in hand behind his back. His character was trying to entice a drunken Johnny Ringo into a gunfight by convincing him that he had a fair hand at winning when in fact, Ringo has almost no chance of surviving the encounter. Holliday says it again with a slightly different meaning at the end of the film knowing he can still best Ringo but secretly hoping that the mortally wounded Ringo would still be able to give the ailing Holiday the warrior's death he desired. When their bodies would be found, they would see that Holiday had time to holster his pistol, revealing that he was the fastest shot. That is why Holiday said 'You're no daisy' (you're not a good man or 'jolly fellow') when he realized he would not get to die with his boots on alongside Ringo.

The term dates back at least to the early 19th century American South and an early example is found in Thomas Thorpe's 'The Mysteries of the Backwoods or Sketches of the Southwest' (1846) where on page 166, it states "Huckleberry above the Persimmons" as a slang term for a 'comparatively easy task that is still beyond the ability of a certain individual' or to merely 'perform a task better than someone else'. Another variant to this is 'above my huckleberry' which is a Southern version of 'above my bend' which means 'beyond my abilities'.  By saying 'I'm your huckleberry', Doc was assuring Ringo that 'you can handle lil' ol' me; we have the same skill level'.

There were actual books on American Slang written during the 1800s and here  are a few references to consult in regards to being above one's huckleberry or other period lingo.  Bartlett's Dictionary of Americanisms (1848 & 1859), Maxmillian Shelle De Vere's Americanisms (1872), or Barriere's Dictionary of Slang, Jargon, and Cant (1889)

[Niederlander]

Wow!  I learned something!

[Trailrider]

IMHO, "I'm your huckleberry!" might have ironically been used like a cowboy would say to his sweetheart if she asked if he would take her to the dance. In this case, Doc was telling Johnny Ringo that if the latter wanted to "start the ball", Doc was more than willing to "dance"!

[Tsalagidave]

IMHO, "I'm your huckleberry!" might have ironically been used like a cowboy would say to his sweetheart if she asked if he would take her to the dance. In this case, Doc was telling Johnny Ringo that if the latter wanted to "start the ball", Doc was more than willing to "dance"!

That's a good way to see it. Though we can no longer ask Kevin Jarre about what compelled him to use this expression when writing the script, I think your explanation fits it best.

-Dave