The Collaborative International Dictionary of English v.0.48:

dishelm \dis*helm"\ (d[i^]s*h[e^]lm"), v. t. [Pref. dis- + helm
   helmet.]
   To deprive of the helmet. [Poetic]
   [1913 Webster]

         Lying stark,
         Dishelmed and mute, and motionlessly pale. --Tennyson.
   [1913 Webster]