The present study investigates the diachronic development of the bipartite negative construction hǎa...mây in Thai, aiming to account for its syntactic and semantic peculiarity. Based on the historical data from the Sukhothai period to Modern Thai, I suggest that the development of hǎa...mây construction relates to the grammaticalization of the expression hǎa NP míɁ dây 'fail to find something'. From around the mid-14th century, hǎa NP míɁ dây came to gain a new function as an irregular

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... existential construction under the pressure of the recession of the anterior negative existential form bɔ̀ɔ mii 'not have, not exist'. This function is especially prominent in the 17th century during which the old negator bɔ̀ɔ shows a continuous decrease in the frequency of use and the newer negator mây was not widely used. When the newer negative existential form mây mii emerged around the early 18th century and eventually prevailed over hǎa NP míɁ dây in the 19th and 20th centuries, hǎa NP míɁ dây gradually lost its function as a negative existential form but survived its evolution into a negative adverbial construction through a syntagmatic change (from hǎa NP to hǎa VP), accompanied by a phonetic reduction (from hǎa...míɁ dây to hǎa...mây ) and a semantic reinterpretation(from 'fail to find something or not exist, not have' to 'not VP as one thought').