Spot On

[FredPuli43]

Why do we say something is 'spot on'.meaning it's dead accurate? What spot was it originally on? Or is it not a phrase taken from some activity but simply an old use of 'spot'?

[Buenchico]

One possibility is suggested here:
http://wiki.answers.com/Q/What_is_the_origin_of_the_phrase_spot_on

[Quizmonster]

I'm certainly not suggesting the above response is wrong, but the word spot, meaning a "small usually roundish mark of a different colour from the main surface" (OED) dates back to the 13th century, which long predates billiards/snooker. In the sense of "a locality of limited extent" (OED) it is of a similar age.
As it happens, the earliest recorded use of the phrase 'spot on' is as recent as 1920 and refers to hitting a military target...ie an exact locality. All-in-all, I'd guess the phrase is just an inversion of 'on the spot'.