This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium. Particularly: “I'm not sure if the form da is still used in any Northern or Southern dialect where it should be native, or is it already completely displaced by the Central đa. It seems that đa first spread to the Northern dialects and then only spread to Southern dialects within the last 130 years or so. There could be some very old speakers in the Mekong Delta that might still use the inherited da.”
The Northern Middle Vietnamese form dĕa is attested in Dictionarium Annamiticum Lusitanum et Latinum (1651), points to the earlier form *-taː. The "modern" Northern-Southern reflex da is attested in Dictionarium anamitico-latinum (1838) and continued to be used as late as works of Hồ Biểu Chánh (a Southern writer) from the 1920s.
This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium. Particularly: “Actually, even later than the aforementioned case, Nguyễn Hữu Quỳ (from Saigon) in his 1971 translation of Tự-Đức Thánh-chế tự-học giải-nghĩa-ca mentions the usage of da by Southern and Central speakers (there was certainly a jumble flip-flopping spread of dialects going on here):
Người Trung, Nam gọi cái dĩa, cây da, người Bắc lại gọi cái đĩa, cây đa. Những tiếng đao (con dao) đai (dẻo dai) hiện giờ vẫn thông dụng tại các vùng quê miền Bắc Trung phần, ngay đến nông dân ở vài miền quê tỉnh Thừa Thiên cũng còn phát âm như thế.
”
Considering that de Rhodes' dictionary was chiefly based on one or several Northern dialects that had undergone lenition quite throughout, the modern form đa without lenition might have spread from a Central dialect less affected by it.
