Chiefly Military. a signal, usually by drum or bugle, that all or certain camp or barracks lights are to be extinguished for the night.
Origin of lights out
First recorded in 1865–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the time when those resident at an institution, such as soldiers in barracks or children at a boarding school, are expected to retire to bed
a fanfare or other signal indicating or signifying this
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012