Green’s Dictionary of Slang

clank n.

also clanker, clink
[echoic; i.e. the tankard, plate or dollar hitting a table]

1. [late 17C–mid-19C] (UK Und.) a silver tankard; thus rum clank, a double tankard.

2. [late 18C] (UK Und.) a silver plate.

3. [19C] (US) a silver dollar.

In compounds

clank-napper (n.) [napper n.1 (1)]

[late 17C–early 19C] a thief who specializes in stealing silver plate.