Here’s a weird one, with a surprisingly straightforward explanation behind it. A Robin Hood’s mile is a mile that seems longer than it actually is.
That’s an expression that dates from the sixteenth century in English (though given the agedness of tales of Robin Hood, it may well have been in use even longer than that).
As well as meaning simply a longer-than-usual mile, this expression can also be used of any journey or route that promises to be short, but either feels or ends up being much longer. There’s also another expression, to go round by Robin Hood’s barn, which is said to refer to any circuitous or longwinded way of getting somewhere.
At the root of both these are the exaggerated stories that once surrounded Robin Hood’s archery skills. Was he really good enough to hit an arrow already planted in a target and splinter it in two? Could he really slay two men with one arrow? Could he really hit anything he was aiming at from a mile or more away?
Myths and tales like these made much of Robin’s abilities—and so a “mile” in Robin Hood’s terms, or in the terms of a story about his talents, was in reality a considerably different distance than the stories would have you believe.