Full butt

headfirst with full force.
- Marryat.

See also: Butt

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in classic literature ?
We ain't nervous folk, as you know well; but when it comes to makin' a week-end visit and finding you've run full butt into the Day of Judgment, it wants a bit of explainin'.
I'd question him on why my brother and not me inherited my mother's petite frame, full butt, and thigh gap.
The Knoxx recoil reduction system works around a cam-activated, spring that is tensioned and then relaxed back into battery as the top portion of the Copstock stock and the full butt of the Compstock move to the rear and compress under the force of recoil.