noun
A bag filled with sand, used for flood control, ballast, or exercise.
洪水に備えて砂袋を積んだ。
We stacked sandbags in preparation for the flood.
鳥の砂嚢は、食べ物をすりつぶす器官だ。
A bird's gizzard (sunabukuro) is an organ that grinds food.
The standard word for 'gizzard', read さのう. 砂袋 read すなぶくろ is a rare alternate reading/kanji combination for the same meaning.
Compound of 砂 (suna, 'sand') and 袋 (fukuro, 'bag'), literally 'sand bag'. The gizzard sense comes from the sand and small stones birds swallow to help grind food.