from The Century Dictionary.
- Get up; go along: addressed to a horse.
Sorry, no etymologies found.
Hiram helped me into the pung, took his place beside me, and threw a conversational "huddup" to the rakish-looking sorrel colt.
Meadow Grass Tales of New England Life Alice Brown 1902
'If that cove's wife could see him -- huddup, then!' said the cabman.
A Duet, with an Occasional Chorus Arthur Conan Doyle 1894