The term lacko is an abbreviation for the longer phrase "lack of nuts" suggesting a man of low testosterone, filled with angst, and generally lacking in most social categories. A lacko is known to lack frequently causing his friends to continually promote benevolent but powerful pranks upon him, in order to keep his lacking in check. The term is thought to have originated from a pool game involving several friends, in which the lacko of the group loudly exclaimed "I've got the small ones!" intending to refer to the pool balls, but subconciously referring to his own nuts.
Dude, did you hear from Lacko?
Naw man, he's really lacking today...
Yep, he sure does lack.