The phrase possibly comes from adjusting a sighting mechanism of a firearm to minimise the discrepancy between where the sight points and where a bullet lands on a target.
Another possible source is from the Cartesian coordinate system in mathematics. In the coordinate system, shapes are generally defined out of an origin (the generic point of reference) labeled O, with an x,y coordinate pair of 0,0.
- To focus one's aim; to zoom in and center on something.
- The pilot zeroed in and launched the guided missile.
- Zero in and find a watermark in the image.
- 1981, USENET 15 Sep 1981  on 08 Dec 2006
- When you invoke the VAX C compiler with -p for profiling it generates an assembly-language call to a profiling subroutine for which I substituted my own heap-checker, and that enabled me to zero in on a heap-violation which was caused by an array-out-of-bounds condition.