We use this phrase when the pressure is on, when the situation is critical or urgent, when the time has come for action, even if it’s difficult. For example, ‘He’s not a very good cook, but when push comes to shove, he can usually get a meal on the table.’
The path seems fairly straightforward from the words ‘push’ and ‘shove’ to ‘when push comes to shove.’ After all, a shove is just a more decisive and aggressive type of push. A shove represents a surge of energy stronger than a mere push.
William Safire wrote a column on the phrase in 1997 where he concludes “a black-English origin for the phrase is pretty likely.” He also found this plausible source of the phrase:
“Other evidence of the phrase’s black origin is a recollection from Norman Pierce of Jack’s Record Cellar in San Francisco of Shove Day, or Bump Day, the traditional Thursday off for domestic servants in the 1920’s, ‘on which blacks accidentally jostled whites in public places, railways, streetcars, etc.’”
However, the earliest OED reference for the exact phrase is from a 1898 newspaper in Georgia: “When ‘push comes to shove’ will editors of the Yellow Kid organs enlist?” An even earlier reference is from United Methodist Free Churches’ Magazine in 1873: “The proposed improvement is about to fail, when Push comes up behind it and gives it a shove, and Pull goes in front and lays into the traces; and, lo! the enterprise advances, the goal is reached!”
Pushing and shoving is also a way of referring to fighting. It’s how a barroom brawl starts: somebody looks at someone the wrong way, and a nasty comment is made. Soon more comments are flying. Then the two antagonists stand up, face each other, puffing out their chests or jutting out their chins in ancient aggressive postures. But will it escalate further? Who’s strong enough to carry out the threat when it goes two steps farther — when push comes to shove?
It could also have its origins in 16th and early 17th century English, during the English Civil War. The phrase is a description of what happens when opposing pike blocks engaged in battle. It was described as push of pike and musket butt. That phrase would have easily percolated through the population since the civilian population was intimately associated with the armies. It could have become ‘push comes to shove’ as that rolls off the tongue so smoothly and aptly describes the soldiers in the pike blocks doing exactly that.
There are similar phrases: when the chips are down, when the gloves come off, in the crunch. But each has a slight but subtle difference in meaning.