Green’s Dictionary of Slang

clinkers n.

[the noise]

1. [late 17C–19C; 1950s] chains and fetters worn by imprisoned felons.

2. [1900s] stairs [creaking stairs or the ‘clink’ of feet].

3. [1940s] (US Und.) handcuffs.

4. [1940s+] (Ulster) the testicles [supposedly ‘clinking’ testicles].