Green’s Dictionary of Slang

(US campus) aristocratic, upper-class.

1841		‘The Four And Ninepenny Hat’ Dublin Comic Songster 103: The dustman, who was fashion’s dunce [...] The castor he’s cast off – ’tis well, / In fashion he’s quite pat, sirs.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 48: pat, adj. Nobby, swell.