"The term "frog" for the indentation on one bed of the brick is a word that often excites curiosity as to its origin.
The most likely explanation is that brickmakers also call the block that is placed in the mould to form the indentation a frog. Modern brickmakers usually use plastic frogs but in the past they were made of wood. When these are wet and have clay on them they resemble the amphibious kind of frog and this is where they got their name. Over time this term also came to refer to the indentation left by them."
( The Mock TurtIe ™ --- Thinks you are a cunt, on, Wed 6 Jun 2012, 22:09, archived)

i seen this, citation needed tho eh, i mean i once found a frog sitting in rain water in a brick in my garden, imagine that

( mongychops, Wed 6 Jun 2012, 22:17, archived)