Learn more. sentence substitute an informal word for... Night-night - definition of night-night by The Free Dictionary. Night-night, echoed Ted, as he wiggled his fingers in a funny fashion at Trouble. How to use night night in a sentence. The Curlytops at Silver Lake | Howard Roger Garis And '' night-night ' and 'daddy' were two-year-old language to Jim. Used to say good night to someone, especially to a child, who is going to bed. An example of nighty night is what you might say to your son as you tuck him into bed. Night night definition is - —used by a child or when speaking to a child as a way of saying 'good night'. YourDictionary definition and usage example. Nighty-night is defined as a baby-talk word that means "good night." y-night Informal interj. night definition: 1. the part of every 24-hour period when it is dark because there is very little light from the….