Green’s Dictionary of Slang

sock adj.

[fig. use of sock n.2 (1)]

excellent, first-rate.

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[US]W. Winchell On Broadway 4 May [synd. col.] Georgie Tapps, another sock entertainer.
[US]J. Weidman Price Is Right 199: It’s sock stuff, Wally.

In compounds

sock frock (n.) [joc./assonant use of SE frock]

(US black) one’s best suit.

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[US]D. Burley N.Y. Amsterdam News 18 Sept. 22: Your deuce of peekers will lamp the sock-frock.