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What you say when you're too lazy to even think about telling someone what's going on, and you're probably already binge-watching cat videos.
'I'll let you know', said by someone who's been offline for a week and has no idea what time it is.
'I'll let you know', after you skipped the group hangout to go get pizza with your crush.
'I'll let you know', when you're already asleep but still replied to the message.