noun
spout of a teapot
Refers specifically to the spout of a Japanese-style teapot (急須).
急須の口からお茶を注ぐ。
Pour tea from the spout of the teapot.
注ぎ口 is a general term for a spout or pouring lip, not limited to teapots.
Compound of 急須 (teapot) + の (possessive particle) + 口 (mouth, opening). The exact historical derivation is straightforward.