English Thing To Be Today: a caution.

 Long ago, when I was young, I'd hear elderly Londoners saying ooh, she's a caution! By which they meant, not that she was a living example of What Not To Do, but that she was full of fun, an extrovert, an original, and the Life and Soul of every party.

The expression was sometimes used sarcastically of someone who either imagined herself wrongly to be the Life and Soul of the party, or who did something staggeringly foolish.

It was used of men, too.

The same phrase in the USA seems to have begun as a caution to snakes (or turtles, or some other creature) and to have there more or less its expected meaning of warning. Even in the USA, though, there's a feeling of original behaviour running through the expression, so perhaps there's a link between the two expressions somewhere.

English Thing To Be Today: a caution. This word comes from the Old French, from the Latin cavēre, to beware.