Flic (French).
A policeman or sergeant de ville. “Une allusion à lʹépée des sergents de ville, ou plutôt aux flèches des archers primitifs” (Raille). Hence “flic-flacs,” thumps and thwacks.
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.