This is the 128th most frequent Vietnamese word.
cổ
"Neck" or "ancient" depending on context.
In this sentence, 'cổ' refers to 'neck', where the necklace ('vòng cổ') is worn.
Anh ấy đeo chiếc vòng cổ bằng vàng.
He wears a gold necklace.
Here, 'cổ' is used to mean 'ancient' or 'historical', describing something old from times past.
Chiếc máy này đã cũ, thuộc thời cổ xưa.
This machine is old, from ancient times.
In this sentence, 'cổ' is used in 'cổ tích', meaning 'folktale' or 'traditional story', denoting something from traditional folklore.