If you decide you are going to scale Mt. Everest next weekend without any training or experience, that would be a foolhardy decision. Use the adjective foolhardy to describe someone who rushes into action without considering the consequences.
Foolhardy is a combination of the noun fool and the adjective hardy, meaning "brave" or "bold." Put them together and you’ve got “foolishly brave.” Someone who is foolhardy throws caution to the wind and takes reckless chances. A foolhardy mistake is typically the result of this kind of impulsive behavior. But foolhardy doesn’t always imply foolishness or stupidity; foolhardy can convey courage and romance, as in the case of a foolhardy passion or desire.