LIS

Someone who sends you LIS is "laughing in silence." While this person wants to LOL at something, they're instead keeping their laughter on the inside, likely to avoid disrupting or offending others.

Most commonly, you'll receive LIS in response to a humorous message you've sent. The person you sent the message to can't laugh aloud now, so they've sent you LIS instead. People also use LIS when describing situations in which they wanted to laugh but (wisely) chose not to.