If something is irresistible, you have to have it, like the plate of gooey chocolate chip cookies sitting in front of a group of dieters who one by one help themselves to a handful.
If you can’t resist something, it’s irresistible. Something is irresistible if it is so enticing, attractive, magnetic, tempting, or alluring that you must have it. You have no control — that’s what all irresistible things have in common. From potato chips to bargain shopping or the urge to send text messages, when something is irresistible, it’s hard to fight.