To The Manner Born
The full quote goes: “but to my mind,—though I am native here and to the manner born,—it is a custom more honour’d in the breach than the observance.” Hamlet is referring to his uncle and his regular nightly custom of getting drunk, dancing, and firing off cannons. Hamlet is saying he’s “to the manner born,” as in he’s a Dane and has participated himself in these revels in the past. But having done so in the past, he now finds it better to abstain. Really he’s just driving another wedge between himself and his would-be uncle, saying “he does this, and I used to that that, but now I don’t because I know better, and therefore I’m better than him.” Classic move, Hamlet.