Source : Webster's Revised Unabridged Dictionary (1913)

Backlash \Back"lash`\, n. [Back, adv. + lash.] (Mech.)
   The distance through which one part of connected machinery,
   as a wheel, piston, or screw, can be moved without moving the
   connected parts, resulting from looseness in fitting or from
   wear; also, the jarring or reflex motion caused in badly
   fitting machinery by irregularities in velocity or a reverse
   of motion.

Source : WordNet®

backlash
     n 1: a movement back from an impact [syn: {recoil}, {repercussion},
           {rebound}]
     2: an adverse reaction to some political or social occurrence;
        "there was a backlash of intolerance"
     v : have an unexpected and undesired effect; "Your comments may
         backfire and cause you a lot of trouble" [syn: {backfire}]