A contraction of "Let's See". Can be used as either street slang, or internet slang. Also spelt "Lessee".
Uber-1337 version would be 13553, although this is not commonly used.
Less should be used as an antonym to mo'. When one needs to emphasize someone's dislike, distaste, or displeasure in terms of something. Mo and less are best when used in close proximity.
If one is using less by itself, as in -- "Less!" it means "Man, that's so lame!!"