The shortest form of the hugely
popular expression "Laughing Out Loud". The evolution from "Laughing Out Loud" to the expression "5" is as shown here:
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.