The Collaborative International Dictionary of English v.0.48:

Helpmate \Help"mate`\, n. [A corruption of the "help meet for
   him" of --Genesis ii. 18.--Fitzedward Hall.]
   A helper; a companion; specifically, a wife.
   [1913 Webster]

         In Minorca the ass and the hog are common helpmates,
         and are yoked together in order to turn up the land.
                                                  --Pennant.
   [1913 Webster]

         A waiting woman was generally considered as the most
         suitable helpmate for a parson.          --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

helpmate
    n 1: a helpful partner [syn: helpmate, helpmeet]

The Devil's Dictionary (1881-1906):

HELPMATE, n.  A wife, or bitter half.

    "Now, why is yer wife called a helpmate, Pat?"
        Says the priest.  "Since the time 'o yer wooin'
    She's niver [sic] assisted in what ye were at --
        For it's naught ye are ever doin'."

    "That's true of yer Riverence [sic]," Patrick replies,
        And no sign of contrition envices;
    "But, bedad, it's a fact which the word implies,
        For she helps to mate the expinses [sic]!"
                                                         Marley Wottel