means Soft, springy, and full of little air pockets — yielding when pressed and then springing back, like a sponge or a slightly underbaked cake.

from From 'sponge' plus the adjective ending '-y.' 'Sponge' came into English through Latin 'spongia,' itself borrowed from Greek 'spongia' (or 'sphongos'), the name for the soft, porous sea creature whose dried skeleton has been used for washing and soaking since antiquity. So to call something 'spongy' is to say it behaves like that absorbent, squashable animal — a word that has soaked up its meaning from the Mediterranean seabed.