The Online Etymology Dictionary has this for pushover:
also push-over, 1900 of jobs or tasks; 1922 of persons (bad boxers and easy women), from push (v.) + over (adv.).
I'm not sure I understand this description. Does it mean the word was used in 1900 to describe jobs and tasks, and then in 1922 to describe people, as in she's a pushover, meaning she's easily led or influenced? And what does bad boxers and easy women mean? I guess it means the word was used to describe bad boxers who can easily fall over at their opponent's punch.