Green’s Dictionary of Slang

(US Und.) to dress oneself up.

1928		(con. 1900s) J. Callahan Man’s Grim Justice 27: The first thing we did was to ‘rag up’ [...] we each spent about one third of our first subway haul investing in new outfits.
1970		Current Sl. V:2 11: Rag heavy, v. To dress well.