Green’s Dictionary of Slang

belly-bumper adv.

also belly-booster, -bump, -bunt, -bust, -buster, -gut

(US) riding face-down on a sledge downhill.

[US]H.A. White ‘A Word-List From Central New York’ in DN III:viii 566: belly-gut, adv. ‘Ride belly-gut’: to slide down hill lying face downward on the sled. Belly-flapper is reported from Northern Vermont, belly-bump from Central Vermont, belly-bumper from near Philadelphia and from Illinois, belly-buster from Missouri, and belly gut and gutsies from vicinity of Boston.
[US]R.W. Brown ‘Word-List From Western Indiana’ in DN III:viii 571: belly-booster or belly-bumper, adv. Flat on the stomach. ‘I’m not afraid to ride down that hill belly-bumper.’.
[US] ‘Gatherings From Newspapers’ in DN IV:iv 305: belly bunt, belly bust, adv. = belly-bump.