The Collaborative International Dictionary of English v.0.48:

Efforce \Ef*force\, v. t. [imp. & p. p. Efforced; p. pr. & vb.
   n. Efforcing.] [OF. esforcier (F. s'efforcer to exert one's
   self), LL. exforciare; L. ex + fortis strong. See Force.]
   To force; to constrain; to compel to yield. [Obs.] --Spenser.
   [1913 Webster]