The Collaborative International Dictionary of English v.0.48:
Nod \Nod\ (n[o^]d), n.
1. A dropping or bending forward of the upper part or top of
Like a drunken sailor on a mast,
Ready with every nod to tumble down. --Shak.
2. A quick or slight downward or forward motion of the head,
in assent, in familiar salutation, in drowsiness, or in
giving a signal, or a command; as, a nod of approval.
A look or a nod only ought to correct them [the
children] when they do amiss. --Locke.
Nations obey my word and wait my nod. --Prior.
The land of Nod, sleep.