Someone who actively decides to do less than their best because either they don't care or they feel the task doesn't deserve the time it would take to do a good job.
"Joe, are you going to put much time into your progress report?"
"Naw, I'm gonna half-ass it, cause nobody will EVER read it anyways".
"Joe, you're such a half-asser".
Someone who tells you half of the truth
When your man or woman leaves out an important detail in a certain situation hince a half-asser