Staving John Cowan:
>A similar oddity is that "Good night" can only be used at parting;
>at meeting, one must say "Good evening" no matter how late it is.
>Consequently, the English lyrics to the Brahms Lullaby begin "Lullaby and
>good night" ("lullaby" is a word of imitative origin meaning a sleeping
>song sung by a mother to a baby); "good evening and good night" would
>sound like meeting and parting in immediate succession.
I've heard a derivation of lullaby from "Lilith abi!", a charm to protect
the child from the demons of the night.
Pete