Archaic botanical term for the inner layer of a fruit's pericarp. Modern Japanese uses 内果皮 (ないかひ).
See also: 甘皮
In old botanical texts, the hard inner layer of a fruit is sometimes written as 甘肌.
The derivation is uncertain; the kanji 甘 (sweet) and 肌 (skin) may reflect an old descriptive term for the inner fruit layer, but this is speculative.