Alteration (after headlong) of arseling, from Middle English*arsling, from Old Englishearsling ("backwards, on the back"), equivalent to arse + -ling. Cognate with Dutchaarzeling, aarzelings, Germanärschling, ärschlings.
1981, John Barth, Giles goat-boy:Heave they did, all unopposed, and tumbled arselong when the bar came about. Even as they rolled and cursed, the whistling petered; ...