Can be used in many different contexts, but basically is a play on "nailing it", or "nailed it". With the emphasis on the fact that the person is "so nailing it" or "super nailing it". Schnailing it.
Can also be substituted for "rocking it" and "having sex". Schnailing it.
Dude, you are so schnailin it right now!
How long did ya'll schnail it for last night?
I totally schnailed that chick in the pool.
Man, you got a great job, a hot girlfriend, and all the friends in the world . . . you are schnailin life right now!