I've believe it came from an old english meaning of the word frog, which was a button hole for a toggle button. So as a natural extention of the meaning, we have a hole to allow the mortar to form a kind of toggle into the brick. So it sort of makes sense to call it a frog. (a hole for a toggle).
Interestingly enough, there's a tool used to trim excess mortar off, when pointing (usually home made). I've always known this as a "Frenchman". Darned if I know why though.
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