Green’s Dictionary of Slang

croaker n.1

[? the harsh, miserable croaking of ravens, supposedly ominous birds; + croak v.1 ]

1. [mid-17C–1930s] a congenital pessimist.

2. [mid-19C+] a whiner or whinger.

3. [mid–late 19C] a beggar.

4. [mid-19C+] (US) one who backs out of undertakings they have promised to perform.

5. [1910s–40s] (US) one who talks too lengthily and too loudly.

6. [1940s+] an informer.