A he or jack ass: so called by a lady that affected to be extremely polite and modest, who would not say Jack because it was vulgar, nor ass because it was indecent.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Jogg-trot * JointNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes