If/when push comes to shove spoken language
When/if no other choice is available; when/if the situation develops in a more or the most unpleasant way; when/if drastic measures are required.
We should come back when you arrive, but if push comes to shove, the neighbor living next door has a spare key that allows you to get into the house.
The study comes to a conclúion that if push comes to shove, there might record 100,000 cases of the disease tomorrow.
This phrase originated in African-American English around the middle of the twentieth century.