Thinking of laughing
Yes, that was very funny ... I'm even TOL.
|NOIRL||No one is really laughing|
|LTMQ||Laughing to myself quietly|
|LOL||Laughing out loud|
|SFF||So freaking funny|
The person who sent you TOL is most likely not actually thinking of laughing. Instead, they're using TOL sarcastically, to say whatever joke you just sent is NF.
On very, very, very rare occasions, someone who sends you TOL might genuinely be thinking about whether they're going to laugh at something. For example, a student sitting through an unintentionally funny lecture might send you "TOL at this professor; I don't think I can stop myself." But 999 times out of 1,000, TOL is a burn, sent when someone finds something intended to be funny to be anything but.