The Collaborative International Dictionary of English v.0.48:

Backslide \Back"slide"\ (b[a^]k"sl[imac]d"; 277), v. i. [imp.
   Backslid; p. p. Backslidden, Backslid; p. pr. & vb. n.
   Backsliding.] [Back, adv. + slide.]
   To slide back; to fall away; esp. to abandon gradually the
   faith and practice of a religion that has been professed.
   [1913 Webster]