The phrase "such-and-such" is a modern English compound formed through the reduplication of the word "such," a common rhetorical device for creating indefinite or placeholder terms. "Such" itself has a deep Germanic lineage, deriving from Old English swylc, a contraction of swa ("so") and lic ("body, form"). This original sense of "of that form or kind" evolved into a general demonstrative pronoun and adjective. The reduplicated construction "such and such" emerged in the 16th century, leveraging the inherent vagueness of "such" to create a convenient placeholder for unspecified names, numbers, or details, effectively meaning "a thing of a certain unspecified kind and another thing of a similar unspecified kind."

This is a fixed compound phrase. It is formed by the reduplication of the word 'such', connected by the conjunction 'and'. 'Such' originates from Old English swylc, swilc, meaning "so formed, of that kind." The phrase functions as a single lexical unit.