Green’s Dictionary of Slang

pad n.5

[SE pad (of paper), on which the payments are listed]
[1940s+] (US Und.)

the regular bribes paid to members of a US police department; usu. in phr. below.

In phrases

on the pad

1. [1940s+] (US Und.) of the police, accepting bribes; of a villain, paying bribes.

2. [1960s] (US) in non-police use, used of any kind of free-loading.