Green’s Dictionary of Slang

mallet n.²

(US black) the police.

1980

E. Folb Runnin’ Down Some Lines 66: The characterization of the police [...] connotes such perceived attributes as brutality (billy for billy club, mallet, whup-a-child).

SE in slang uses

In phrases

on the mallet(t) [they are knocked down/out]

of goods, taken on trust.

1835		G. Kent Modern Flash Dict.
1846		Swell’s Night Guide 127/1: On the mallett having goods on trust.