One of the mysteries of the English language finally explained.
Used in representations or imitations of Japanese and (by misidentification) Chinese speech, especially to express affirmation, acknowledgement, understanding, or realization: ‘Ah!’; ‘So!’; ‘I see!’.
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 [conjunction].
ah so/ɑː ˈsəʊ/
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