Are expletives allowed on SE sites?
I saw a comment that contained offensive words (e.g., fu**er).
How do I report that? Is this acceptable behavior on Stack Overflow?
Hover over the area next to the comment (where you would normally vote it up), and there is a flag icon to notify a moderator about it.
As to whether that is acceptable, I would say no, it's offensive language. More so if it's directed at someone in particular. People may have differing opinions on certain words, it's subjective.
Also, a link (or a screenshot) might help so others can check it out. Right now, it's hearsay.