This quote comes from an insult. The Earl of Kent is set to fight with Oswald and the two draw swords.
Kent sees Oswald's true nature and regards him with utter contempt...
When the fight is broken up, Kent says to Oswald, "Nature disclaims in thee; a tailor made thee!"
The implication here is that Oswald does not possess the natural bravery of a man. He is empty of natural, manly faculties. In this way he is like a suit of clothes with no inner human qualities, only the outward trappings of humanity.
Then Oswald questions Kent's meaning, "Though art a strange fellow. A tailor make a man?"
At this point, Kent amplifies his insult, suggesting that a tailor or stone cutter who had only been practicing his trade for two hours would have done an equally poor job of constructing the outward image of a man.
Kent's insult is double here. He insults Oswald's qualities in terms of courage and then in terms of looks. For his part, Kent represents a nobility and integrity in the play that other characters tend to lack.
Kent and Edgar demonstrate their nobility in their actions.
Coming from Kent, the insult given to Oswald carries extra weight.