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]

WordNet (r) 3.0 (2006):

backslide
    v 1: drop to a lower level, as in one's morals or standards
         [syn: lapse, backslide]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "backslide":
   cock, defect, degenerate, desert, err, fall, fall again into,
   fall astern, fall away, fall back, fall behind, fall from grace,
   get behind, go astray, go backwards, go behind, go wrong,
   have a relapse, jerk back, lapse, lapse back, lose ground,
   pull back, recede, recidivate, recur to, regress, relapse,
   retrocede, retroflex, retrograde, retrogress, retrovert, return,
   return to, reverse, revert, revert to, sink back, slide back, slip,
   slip back, tergiversate, trip, turn, yield again to



Easton's 1897 Bible Dictionary:

Backslide
   to draw back or apostatize in matters of religion (Acts 21:21; 2
   Thess. 2:3; 1 Tim. 4:1). This may be either partial (Prov.
   14:14) or complete (Heb. 6:4-6; 10:38, 39). The apostasy may be
   both doctrinal and moral.