It’s not baited breath; it’s bated breath.

It’s not that your breath has some kind of bait attached to it; the idea is that your breath is held or restrained. Bated is a shortened version of abated which means to lessen.

It’s not free reign; it’s free rein.

This is a straightforward misinterpretation and an understandable mistake. We have a notion of reigning kings and queens doing as they please, that is, having free reign. But the rein in this expression is the strap used by a rider to control a horse. To give a horse free rein is to allow it to run as it will, without restriction.

Apparently, the misspelling free reign dates back to the 1880s, so it’s been around for a very long time.

It’s not shoe-in; it’s shoo-in.

A ‘shoo-in’ is a venture that is certain to be accomplished. A horse that is expected to be a sure winner could be described as a shoo-in, or a candidate standing for election in a safe seat. It seems to have originated in horse racing in the 1900s when corrupt jockeys would select a long shot to beat the faster horses which would then be shooed-in by the others.

It’s interesting to speculate why the shoe version came to appear from time to time. It could be connected to door-to-door salesmen who might try to get a sale by putting a shoe in the door. Perhaps more likely, there may have been a link to shoe-horns, which are used to make it easier to put shoes on.

 It’s not slight of hand; it’s sleight of hand.

Sleight, which is pronounced the same as slight, is no longer a widely used word and its meaning is not commonly known. Sleight means trickery or cunning and that makes sense of the expression. Conjurers and magicians use trickery and deceit to fool their audience and, when they do it using their hands, that’s sleight of hand.

It’s not wet your appetite; it’s whet your appetite.

Here’s another of those words that are hardly used nowadays: whet means to sharpen, often on a whetstone, or put a fine point on something. So, whet your appetite means to sharpen your desire for something; this could often be for food, but it could be for books or music or films etc.

Confusingly, although we whet our appetite, we wet our whistle. In that case our whistle means our voice or throat and wet has its usual meaning. So to wet our whistle is to have a drink.