1. [late 19C+] (Aus.) a lazy person, one who would ‘sooner’ lie around than work or, in context, fight.
2. [late 19C+] (also just as soon dog) a mongrel or any dog that would ‘rather feed than fight’; the same applies to cats; also attrib.
3. [20C+] (US) an illegitimate child [born too soon for the wedding].
4. [1900s] (US Und.) one who takes things for granted; a mistaken optimist [they feel optimistic too soon].
5. [1930s] (Aus.) a confidence trickster.
6. [1930s–40s] (US black) a cheap and badly made object; a dirty and unkempt person.
7. [1950s] (US) a choice.
8. [1980s] (N.Z.) an ill-behaved, lazy horse.