All these responses are silly assumptions!
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.
Ringo: Wretched slugs, don't any of you have the guts to play for blood?
Doc: I'm your huckleberry.