Why is titmouse a name for a type of bird if the word mouse is in it? Well, turns out it has nothing to do with mice at all! When it was first borrowed in the fourteenth century, the word was actually spelled titmose. By the sixteenth century, however, people decided that mose looked like a weird suffix, so they changed it to look like the name of another common animal (this shift also marked the change in the plural from titmoses to titmice, again modelled off the rodent). The first part of titmose, tit, still exists as a word meaning "small bird" today. It has no connection to the vulgar word for "breast" and is most likely imitative of a tapping sound that was associated with the bird. The second part, mose, comes from Old English mase, which also referred to titmice, making the tit prefix completely redundant. In Proto-Germanic, this was maison or maiso, and beyond that the etymology is unknown.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd