When someone says he/she will do something and then does not do it because of of laziness, irresponsibility or worthlessness. Can also mean when someone needs to do a task within a certain timeframe and then gives notice of failure after the time has expired.
Sean: Hey Jeremy, can you get me my tickets before 6:00pm tomorrow?
Jeremy (at 6:30pm the next day): I will not be able to get you your tickets.
Sean: Damn, that Jeremy is always swiffing out...