His lamebrained idea to jump off the roof ended in disaster.
Her lamebrained decision to quit her job without a backup plan left her in a tough situation.
Their lamebrained plan to drive cross-country without a map or GPS led them to getting lost in the middle of nowhere.
Making such a lamebrained mistake cost him the promotion he had been working towards.
His lamebrained move to invest all his savings in a risky venture proved to be a costly error.