To renege is to go back on your word or fail to keep a promise.
Not quite lying, reneging is more a sin of omission — failing to do what you said you would. The Latin negāre means "to deny," so by reneging on your word, you are denying someone whatever you promised them. In card games, you are said to renege if you play against the rules. To renege may be wrong, but it's not necessarily a punishable offense (unless you put that promise legally binding in writing). Still, it certainly doesn't make you look good!