It's about the division between "fighting clean" and "fighting dirty". Yes, hitting someone wearing glasses may make the glass shatter and blind him permanently, and that would win you the fight... but it's a terribly shameful way of winning; it's taking advantage of a weakness and almost akin to cheating. That is fighting dirty. ;) Yep, the theory of fisticuffs is that one must still "fight fair" even if you are actually pummeling each other like barbarians.

I've never heard of this expression. Sorry. I have heard, "Never hit a man when he is down". It means, when a person is already feeling low or in a bad situation, do not do or say anything to them that might make it worse. But this is not the phrase you originally asked about. Yes, I know... maybe someone else can help you with that.

i didn't know this expression too. i searched , but couldn't find the meaning . i just found it completion which is : Never hit a man with glasses. Hit him with something bigger and heavier or Never hit a man with glasses. Hit him with a baseball bat.

It means 'don't pick on those who are weaker than yourself' Those who wear glasses are often stereotyped to be weaker, more nerdy, etc. It's a broad, inaccurate generalization, but that is where the the saying comes from.