According to Online Etymology Dictionary, "1899, U.S. military slang for 'Filipino' during the insurrection there, probably from a native word, or imitative of the babbling sound of their language to American ears (cf. barbarian). The term goo-goo eyes 'soft, seductive eyes' was in vogue c.1900 and may have contributed to this somehow. Extended over time to 'Nicaraguan,' 'any Pacific Islander' (World War II), 'Korean' (1950s), 'Vietnamese' and 'any Asian' (1960s)."
Online Etymology Dictionary