Tis is a well know Christmas song, whose lyrics were written by 19th century British poet Christina Rossetti. Today it has been recorded by various artists, including those of both genders. Whether intentionally or by accident, the lyricist in this case wrote words that can be self-insulting when sung by a man, but not when song by a woman. When the subordinating conjunction "if" introduces a clause with the verb in the subjunctive ("were" in this case"), the clause refers to conditions contrary to fact. So the singer is here implying that he or she is not a wise man. If the singer is a man, that unavoidably suggests that he is not wise. But for a woman singer, the clause's contrariness to fact is satisfied merely by the fact that she isn't a man. So there is no necessary suggestion that she isn't wise!