bí đao

Etymology
It is not clear what the second element is. In Nhật dụng thường đàm (日用常談, 1827) and the Dictionarium Anamitico-Latinum (1838), bí đao was spelled as and  respectively. In both cases, the second element is spelled with (SV: ), which makes little sense if taken literally ("knife gourd" or "sword gourd"), so it is most likely a phonogram used simply for phonetic value (just like the adjacent phonograms  and  used to write ).

Considering that winter melon/wax gourd is commonly used to make sweets (Đại Nam quấc âm tự vị (1895) mentions ), the second element could be related to 🇨🇬 atɛːw, 🇨🇬, 🇨🇬, all means "sugarcane"; if so, probably developed already into "sugary sweet" when this compound was coined, then became obscure.

Noun

 * 1) winter melon