I only know this word (serif) as being related to fonts. Does it have another meaning?
Sometimes, in movies, it is used as a way of addressing a friend (a boy by another boy), so you could have heard it in movies/ series more often than we use it in real life.
The sentence used in the first question says "Vreau o bere, boss!“ (I want a bear, boss!), which is also rude, but one could use is when talking to a friend.