(obsolete, idiomatic) To get a positive test result from an old-fashioned pregnancy test.
"What about her?" "She killed the rabbit."
Origin of kill-the-rabbit
Before the days of instant pregnancy tests, a woman's sample was injected into a rabbit. There were several varieties of tests; one allegedly would kill the rabbit if the pregnancy hormone was present in the sample (most accounts indicate the rabbit was killed and dissected either way.)