The Collaborative International Dictionary of English v.0.48:

Unbowed \Un*bowed"\, a. [Pref. un- not + bowed.]
   Not bent or arched; not bowed down. --Byron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unbowed
    adj 1: erect in posture; "sit straight"; "stood defiantly with
           unbowed back" [syn: straight, unbent, unbowed]
    2: not forced to bow down to a conqueror

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "unbowed":
   arrowlike, dead straight, direct, even, flat, horizontal,
   in a line, level, lineal, linear, rectilineal, rectilinear, right,
   ruler-straight, smooth, straight, straight-cut, straight-front,
   straight-side, streamlined, true, unbeaten, unbending, unbent,
   unbroken, unconquered, uncurved, undefeated, undeflected,
   undeviating, undistorted, uninterrupted, unquelled, unsubdued,
   unswerving, unturned, unvanquished, upright, vertical