The compound "washout" originates from the literal geological phenomenon where water washes away soil, leaving a gully or eroded area. The verb phrase "wash out" was used from the 16th century to describe this cleansing or scouring action. By the late 19th century, it began to be used figuratively in engineering and military contexts to describe the failure of a structure or plan, akin to it being eroded away. This extended to describe a person who fails to meet standards, as if they had been "washed out" of a program.

Structure: wash (to cleanse with water) + out (external, completely) wash: From Old English wæscan, meaning "to cleanse by washing with water". Its functional role is the core action. out: From Old English ūt, meaning "out, outside, away". Its functional role is an adverbial particle indicating completion, removal, or external result.