Merriam-Webster's Collegiate® Dictionary, 11th Edition
NOD
I. verb (nodded; nodding)
Etymology: Middle English nodden; perhaps akin to Old High German hnotōn to shake
Date: 14th century
intransitive verb to make a quick downward motion of the head whether deliberately (as in expressing assent or salutation) or involuntarily (as from drowsiness), to incline or sway from the vertical as though ready to fall, to bend or sway the upper part gently downward or forward ; bob gently, to make a slip or error in a moment of abstraction, transitive verb to incline (as the head) downward or forward, to bring, invite, or send by a nod , to signify by a nod , nodder noun
II. noun
Date: circa 1541
the act or an instance of nodding , an indication especially of approval or recognition