I think what you heard was the expression "on a whim". A "whim" is a sudden idea made without much thought. If you do something "on a whim" then it means you do it without giving it any thought, simply because the idea occurred to you. You might, for example, see a book and decide to buy it immediately, without working out whether you can afford it.
A "wimp", by the way, is a person, usually a man, who is weak and unable to defend himself or fight back.