うなずく
うなずく · unazuku · JLPT N2 · verb-godan
to nod
unazuku = una (nape of the neck) + tsuku (to thrust): dip the neck
Old Japanese una 項 means the nape of the neck, and tsuku 突く means to thrust or poke. To nod is to push the back of your neck forward and down. The ts softened to z when the two parts joined, giving unazuku.