I read somewhere that it was the sign of slavery, but I cannot find anything other than "fashion" as a reason.
Can you explain your question better? Men where? When?
Hair always depends on the culture, but I think you're perhaps referring to the Vikings. They kept their hair long as a sign of strength, and, yes, only slaves had it short.
But again, it's a matter of culture. Most Roman nobles had short hair, and having long hair was something the barbarians did.
hi! you may be interested in some of the previous responses in this section of the FAQ (link on sidebar):
On men with short hair, women with long hair, and vice versa