Shoo, shoe. Stop photobombing the picture! (A shoo-in for a photography award? Probably not.)
Are we talking about wedging a foot into a doorway or about shooing something in a certain direction? Do you know?
When this expression is spelled wrong, it’s almost justifiable. To “get a foot in the door” is a common idiom, and this one could be related… But it’s not.
“Shoo-in” is the correct form, first appearing in the early twentieth century in regards to horse-racing. A horse was a “shoo-in” if it was a “sure thing.”
If you remember the old song “Shoo, fly. Don’t bother me,” you can understand this use of “shoo.” You want to shoo a fly away from your picnic. However, you could also shoo it toward something. Shoo it toward the finish line perhaps? This may not be a common case with flies, but with horses, politicians, and so much more, being a “shoo-in” is a familiar turn of phrase.
Maybe you don’t write “shoo-in” often, but when you do, make sure you leave your feet out of it.