headfirst
1. Literally, with one's head at the forefront; leading with one's head. He tripped on a crack in the pavement and fell headfirst down a manhole.
2. By extension, recklessly, brashly, or thoughtlessly. Before you go rushing headfirst into some half-baked business venture with these guys, take a minute and think about whether something like this could actually work. My brother was forever going headfirst into really serious relationships that would suddenly fizzle out a few months later.