This is a mistake. The expression should be 'second nature'. If something is second nature to you it is natural and instinctive. 'Second hand' is used to describe an object that has belonged to someone else before you, like when you buy a second-hand car rather than a new one. This expression makes no sense at all in the sentence you have given. Where did you find this sentence?

I agree with Su Ki. I think the writer must mean second NATURE.

I, too, agree with Su.Ki. "Second hand" simply doesn't make sense here, while "second nature" makes perfect sense.