This is the 796th most frequent Vietnamese word.
mũ
"Hat" or "cap."
Here, "mũ" means "hat" and refers to a piece of headwear.
Tôi mua một chiếc mũ mới hôm qua.
I bought a new hat yesterday.
In this context, "mũ" is used as part of "mũ bảo hiểm," meaning "helmet."
Mũ bảo hiểm là cần thiết khi đi xe máy.
A helmet is necessary when riding a motorbike.
The word "mũ" again refers to a protective headwear worn against sunlight.