I've always wanted to write in flagrante delicto, because it sounds so deliciously bad. And this very day my dog, Penny, gave me the chance to legitimately use the phrase. (She was only eating stinky compost in the garden - not the other activity we think of when we use this phrase.)
It means literally, in Latin, while the crime was blazing, and Merriam Webster gives the first known use as 1772. I wonder what someone did in that year to cause the creation of this great expression. Maybe went around setting haystacks on fire?
The Macquarie defines this meaning as the transgression blazing.
I guess the nearest English expression would be red handed. But I couldn't use that for my dog, so the nearest canine equivalent might be black-mouthed.