"Estimated Time of Sleep" abbreviated.
Akin to ETA
(estimated time of arrival), etc.
Sounds more official and professional when said aloud.
Usually posed as a question toward someone who --despite the late hour-- still has a lot of things to get done before even thinking about going to bed.
Friend1: "What are you doing up so late?"
Friend2: "I'm working through a 10-page paper that's due tomorrow and just started a load of laundry."
Friend1: "So what's your ETS tonight?"
Friend2: "Aiming for 4 AM, have to get to work by 10."
"ETS unknown; imminent all nighter