Since the overused LOL has become cliched, UCM is making its way in. Literally, the acronym is short for "Unexpected Chuckle Moment." UCM should only be used in moments where one actually bursts out in a chuckle, ideally in surprise.
Alice was in her boring work meeting when she read an email from her sister, causing an unexpected - and unwanted - giggle. She responded with, "UCM! I couldn't hold it in!"
You complete me :)
-ily, ucm <3