Verb. To irreparably screw something up as a result of taking too many shortcuts. May also be used to describe the act of putting something together with the minimum amount of effort needed so that it works, but barely and not for long. Also for use when describing the art of stringing somebody along by showing the minimal level of competence needed to demonstrate that a task could possibly be achieved.
"If you didn't mullins that brake repair by using the Honda rotors sitting in your garage instead of the correct ones for your Toyota, we wouldn't have crashed!"
"Your back door won't shut all they way. Maybe you could just mullins it shut with some duct tape?"
"I was ready to fire that lazy bastard, but he mullinsed his way through it again by turning in those fifteen past-due reports!"