i.e. oat stealer. Hosteler was originally the name for an inn-keeper; inns being in old English styled hostels, from the French signifying the same.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Horse’s Meal * Hot PotNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes