Time today to look at the origins of the word “thumb”. This simple word goes back a long way, at least to the 700s in English, where we find it in the form thuma (þuma). As you can see, there is no b on the end. This is a much later addition, having first surfaced towards the end of the 13th century. It doesn’t take much imagination to see how the b crept into pronunciation when the word consisted of two syllables (thuma), and it was a natural progression following the m.
So where did thuma come from then? Well, we got it from the West Germanic root *thumon, from which there are cognates of thumb in a number of other languages in the Germanic family, such as German and Dutch.
The theory is that it came from the Proto-Indo-European root *tum, or perhaps *teu which may have meant “to swell”. Today’s meaning could have come about through the size of the thumb relative to the other fingers, and its larger top half – the swollen digit, as it were. Following this theory a little further brings us to the word “thigh”, an etymological relative of “thumb”, perhaps the swollen part of the leg in comparison with the calf.
Another word more obviously related to “thumb” is of course “thimble”, whose b is likely to have been added in a similar way to the b on thumb.