Does the invitation you’ve just received in the post contain all the “earmarks” of promising a good time? Have you “earmarked” a portion of your savings for a rainy day?
Earmarks, originally, were just that: small notches or punches made in the ear of a sheep or cow that contained valuable identification information about the animal. Who owned it, which flock it belonged to, whether or not it was a good breeder/milker/stud, and so on. Thin ear skin, once cut, will not grow back together again, so the coded marking is quick, permanent, and as descriptive as the owner wishes to make it.