Green’s Dictionary of Slang

p.d.k. n.

[abbr. polyester double-knit, the epitome of unfashionable tailoring]

(US campus) someone who is out of fashion.

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[US]Eble Sl. and Sociability 37: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] PDK (polyester double knit) ‘someone who is out of date or out of touch’.