1920s; earliest use found in Everybody's Magazine. From Japanese ā sō, interjection signalling attention or understanding in conversation (also a sō, assō; shortening of ā sō desu ka?, lit. ‘ oh, is that so?’ from ā oh + sō so + desu, (polite form of da to be) + ka interrogative particle); the reception of the Japanese interjection in English has probably been influenced by association with ah and so conj..