Etymology tree
Proto-Indo-European *nókʷts
Proto-Germanic *nahts
Proto-West Germanic *naht
Proto-West Germanic *nahti
Old English niht
Middle English nyght
English night
Proto-Germanic *-j-, *-ij-
Proto-West Germanic *-i, *-ī
Old English -iġ
Middle English -y
English -y
English nighty
From night + -y (suffix forming colloquialisms).
Meanings
(childish) Good night; abbreviation of nighty night.