What Does "Backlash" Mean?

It's the "slack" between a driving member and the thing being driven. For example, in a motor/geartrain/wheel system, it's how far the motor can turn in each direction before the wheel moves.

In maintenance backlash may mean excessive wear, a breakdown looking for a place to happen, Time to replace or repair. Spend some $ or L&TC.

Decision must be made : Spend a c now or a $ later.

Inaccuracies om precision machines.

http://en.wikipedia.org/wiki/Backlash_%28engineering%29

Depends upon the context. You already have some good technical replies. It can also mean the repercussions of something done, or the neck injury sustained (usually) in a car crash. The traditional form of navy punishment also exists as well ( ie Cat-o'-nine tails type thing).