Etymology 1

Use traced to U.S. Marines in Philippines in early 20th century.
Earliest recorded example is dated 1920.

Noun

1. A person from East or Southeast Asia, in particular a Vietnamese, Filipino, Japanese, Korean person.

Dictionary notes

In the US, gook refers particularly to a Vietnamese person in the context of the Vietnam War, and particularly to the Viet Cong. It is generally considered highly offensive, on a par with nigger. In a highly charged and nuanced incident, Senator John McCain famously used the word publicly to refer specifically to his former captors, then apologized in deference to the Vietnamese community at large.

Folk etymology suggests that during the Korean War, young Korean children would point at U.S. soldiers and shout "미국" (mee-gook), the Korean word for "America". Soldiers heard the word as "Me Gook", as if the children were defining themselves as "Gooks." The soldiers proceeded to use that term to refer to the Koreans. The word 국 (國, gook) itself simply means "country". This explanation ignores the fact that there are many examples of the word's use that pre-date the Korean War.

Noun

1. Grime or mud