Green’s Dictionary of Slang

(US, Hawaiian) a Japanese-American from the USA rather than from Hawaii.

1953		in Morimoto Hawaiian Dial. Eng. (1966) 111: ‘Kotonk’ is an appellation fixed on Mainland Nisei’s by their Hawaiian brethren. Its origin is attributed to a wise-acre from Hawaii who claimed if one tapped on the skull of a Mainland Nisei, the resulting sound would be hollow ‘kotonk’ like that of a coconut [DARE].
1966		Morimoto Hawaiian Dial. Eng. 111: A mainland girl of Japanese ancestry related...When I first started going to this high school, I wondered why the boy I sat next to never spoke to me. We have since become friends so I asked why he wasn’t friendly, and he jokingly replied, ‘I didn’t want to be caught talking to a stupid “kotonk”. ’ [DARE].
1972		Carr Da Kine Talk 93: Two humorous terms used during World War II were Buddhahead [...] for Japanese-Americans born in Hawaii, and Kotonk, for those born on the mainland of the United States [DARE].
1995		N.Y. Times 19 June 6/4: The Hawaiians called the mainlanders ‘Katonks,’ meaning hollowheads, Mr. Oka said, and they in turn were called ‘Buddhaheads’ as ridicule for their pidgin English and more traditional ways [DARE].