Internet slang term, derived from the popular "LOL" (laughing out loud).
Usage of LUL is typically reserved for situations which make you feel uneasy.
Guest2658 : In high school, I was rated 'most likely to be in jail before graduating', because I kept beating up the janitor and stealing his lunch money.
Replacement internet lingo for 'lol' (laugh out loud)
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.
OR
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.
Jack: omg my mums a fricken hoe bag bitch
John: lul
or
Jake: ffs this kid last night came on our vent channel and he kept tk'in me n was like lulululululul