This phrase has been used in cricket since at least the 1980s. According to Wikipedia, the English cricket journalist and former player Peter Roebuck used it during the 1986–7 Ashes series, describing it as an "American" phrase.
In cricket it refers to a narrow line just outside the batter's off stump. Bowlers aim to pitch the ball down this line, making the batter unsure whether to play it or not – often resulting in an edge to the wicket–keeper or the slips, or even striking the off stump itself. (I'm guessing that in America it's used to describe an equivalent line in baseball.)
The phrase is apparently used in football to describe a line along which wingers aim to deliver a cross, making the goalkeeper unsure whether to come out for it or stay on his line. But you might argue that it's less appropriate in this context, as the aim is to deliver the ball to a specific area of the pitch, from anywhere in a much larger area – rather than along a particular line. In any case: despite its possible American origin, I'm pretty sure it was well known in cricket before it was ever used in football, and IMHO the inclusion of the word "principally" in the question makes it unneccesary to accept football as an answer.
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