LOL means "Laughing out loud" and is a common acronym used in chatting everywhere.
LOL (or lol), written in 1337 is 101.
101 in binary is equal to 5 in decimal.
So, LOL = 5
BROTHER: And the Lord spake, saying, 'First shalt thou take out the Holy Pin. Then, shalt thou count to three. No more. No less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then, lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in My sight, shall snuff it.'
ARTHUR: Right! One! Two! Five!"
GALAHAD: Three, sir!
2) the number of fingers on one hand, or a hand slap involving the hand and this many fingers
3) short for "five minutes"
4) the cops...short for 5-0 (five O)
Nice job on the 7-11 man, give me 5! Now let's hit up this Dunkin' Donuts. Oh shit, the 5 are in there! 5 of them, dogg! Better wait 5.
1. Laughing Out Loud - (Original)
2. lol - (Original -> Acronym)
3. 101 - (Acronym -> 1337 Acronym)
4. 5 - (Binary -> Decimal)
By using "5" instead of "lol", 1337 typists save 66% of their time, thus the need for the expression "5".
Typically a situation that warrants a "5" is funnier than a situation in which "lol" will suffice.