Resist + -ible = Resistible, something that can be resisted. It's a real word, but not common. You might think you could prefix an "un-" in front to make the opposite, but you can't. In this case, the prefix is "ir-" Irresistible, something that canNOT be resisted. It often is used in connection with something tempting. "That chocolate cake is irresistible." In the musical "Damn Yankees," the character Lola, trying to seduce Joe Hardy, sings "I'm irresistible, you fool." A old and well-known philosophical riddle asks "What happens if an irresistible force meets and immovable object?" You get stuck for a while until you realize the solution is simple--just because you can name something doesn't mean it is real. If there is an irresistible force, it is impossible for there to be such a thing as an immovable object, and vice versa.

Andrea Corr is irresistible: https://www.youtube.com/watch?v=cZInjYRUOwc

There are many dictionaries online...Is there something specific about a definition that you do not understand?