Green’s Dictionary of Slang

string and top n.

also silk and top
[rhy. sl. = cop n.1 (1)]

(US) a police officer.

[US]T.A. Dorgan in Zwilling TAD Lex. (1993) 79: String and top, that’s a cop.
[US]A.J. Pollock Und. Speaks n.p.: Silk and top / String and top, a cop.
[US]Berrey & Van den Bark Amer. Thes. Sl. (2nd edn).