Nah, I’m just pumped, I bought some shit from the thrift shop

from Macklemore and Ryan Lewis (Ft. Macklemore & Wanz) – Thrift Shop Lyrics on Rap Genius

He’s explaining the overconfidence/ arrogance exhibited in the previous line is because he just got a lot of stuff from the thrift shop. He’s expressing that the first line is how he feels after copping all the fly shit from the thrift shop but he wouldn’t, really, walk into a club and deliver such a “dickish” line. “Nah” here indicates that he’s kidding about the first verse.

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