Literally: scoop – child – determine – measure
Alternately: Assessing everything based on the same criteria, regardless of whether this is appropriate or not. Inflexible; clinging to a single set of rules no matter what. Like trying to measure all sizes and distances by comparing them to the handle of a ladle.
In contrast to Sunday’s kotowaza, the 杓子 in this case apparently refers specifically to a “dipper” called a 柄杓 (hishaku), which takes the form of a sort of cup at the end of a stick. While an image search suggests that modern hishaku have straight, dowel-like handles, my sources say that historical ones had bent handles, making them almost useless as measuring-sticks.
This compound may also be expanded into a normal phrase as 杓子を定規にする (shakushi wo jougi ni suru), “to make a measuring-stick out of a ladle.” 四角四面 is considered a synonym.
This phrase apparently comes to us from the writing of Tokugawa-era scholar 三浦梅園 (Miura Baien).