What 腭 means
It means the palate: the roof of the mouth.
A Japanese dictionary cites 腭 in 十巻本和名抄, an early Japanese wordbook, around 934. That is a written attestation, not a creation date.
Japanese sources also record the reading あぎ (agi) in references to fish gills and in medieval Noh voice-training texts.
腭, 齶, 颚, or 顎?
腭 and 齶 name the palate; 颚 and 顎 name the jaw.
齶 is a documented alternate written form of 腭. The jaw is the bone that holds the teeth; the palate is the roof inside the mouth.
A technical anatomical term for the roof of the mouth, used in descriptions of the hard and soft palate.
Do not use 颚 or 顎 when you mean the palate.
Don't confuse it with
2Questions people ask
1Why does 腭 show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type 腭
腭\816DCodes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.