ETYMOLOGY
from sit (vb.) + upon (prep.), after to sit upon
EXAMPLE
“… I need scarcely say that he kept a tiger, and that the tiger was a perfect model of a brute. He wore a sky-blue coat with silver buttons, a pink-striped waistcoat, green plush sit-upons, and flesh-coloured silks in-doors; out of doors the lower garments were exchanged for immaculate white doeskins, and topboots — virgin Woodstocks on his hands, and a glazed hat upon his head with forty-two yards of silver-thread upon it to loop up the brims to two silver buttons. …”
From: Peter Priggins, The College Scout
By Theodore Hook, 1841
Lexicophilia 