In 1 Corinthians 11 verse 4, Paul says that if a man prays or teaches with his head covered, he dishonours his head, Christ. Since he says in verse 15 that a woman's long hair is given to her as a covering then it must be true that a man who grows his hair long is equally covered. Since he dishours his head, Christ, if his head is covered, it is therefore a sin if he has long hair or as he says in verse 14, it is a "shame" to him. Vine's New Testament Greek dictionary says that this word, 'altimia', means dishonour, reproach, shame, vile.
I hope you have found this helpful.
God bless,
Mike