ガク, あご, あぎと

jaw, chin, gill

JLPT0 G7 S18 F5000

As per # (cross, especially, cross over, or back and forth) + 頁 head → *jaw* (enabling the crossing back and forth of the upper and lower teeth in chewing; compare 鰐). For "jaw," compare 頤 and 齶.

[Full Kanji Etymology List]