An act which requires a great deal of precision. Used in engineering and building trades, usually in the negative. Example: Pete: Hey, Joe, how long is that two-by-four? Joe: Why Pete, that two-by-four is 15 feet, 6 and 37 sixty-fourths inches long. Pete: Easy, Joe, we're not building a watch here.
BUILDING A WATCH
Meaning of BUILDING A WATCH in English
Slang English vocab. Английский сленговый словарь. 2012