bellyboard

[ bel-ee-bawrd, -bohrd ]
/ ˈbɛl iˌbɔrd, -ˌboʊrd /

noun

a small surfboard, usually 3–4 feet (0.9–1.2 meters) long, for riding waves on one's belly, sometimes used in conjunction with swim fins.

Origin of bellyboard

First recorded in 1960–65; belly + board