fatty underbelly of a fish
Archaic term for the fatty belly portion of a fish, now largely replaced by 砂ずり or 砂摺り, which also refer to a bird's gizzard. The reading つちすり is also archaic.
See also: 砂ずり (すなずり)
Old documents record that the fatty underbelly of fish (sunazuri) was highly prized.
Common spelling for the fish belly fat sense, also used for the gizzard of a bird.
The kanji 腴 is composed of 肉 (flesh) and 臾 (a moment, but here likely phonetic), originally meaning 'fatty meat'. The reading すなずり is of uncertain origin, possibly related to 砂 (sand) and 摺り (rubbing), but the exact derivation is unclear.