Internet slang term, derived from the popular "LOL" (laughing out loud).
Usage of LUL is typically reserved for situations which make you feel uneasy.
You : LUL
Some weirdo's say lol in real life - and if you do an impression of someone saying 'lol' out loud you write it as lul.
if you are talking about a n00b who was lol'ing it up all day you would say how he kept lul'ing all the time.
Jake: ffs this kid last night came on our vent channel and he kept tk'in me n was like lulululululul
See definition of Trolling - The art of deliberately, cleverly, and secretly pissing people off, usually via the internet, using dialogue.