ngu giống như con bò

Dumb as a Cow

This is a Vietnamese folk metaphor.  The informant learned this metaphor when she was a child.  Her parents, first generation Americans, were very careful not to teach her or her brother any insults or swear words in Vietnamese.  However, the informant heard her mother using this expression one day when her mother was frustrated at a bad driver in traffic.  The informant and her brother have continued to use this insult as it is the only insult in Vietnamese they know, to the reluctant tolerance of her parents.

The informant believes this metaphor is offensive because it is very offensive to compare people with animals in Vietnamese culture.  It used to describe somebody who does something stupid.  I agree with this analysis.