December is the 12th month, despite the fact that its name means the ‘tenth’. It derives from Latin decem ‘ten’, as December was the tenth month of the old Roman calendar, which began with March.
I can spend hours looking for etymologies, and have compiled a list of my favourite resources.
The advantage is that you can search in many languages. The disadvantage is that being a compilation, it can be unreliable or plainly wrong.
Yes, it is in Estonian, but invaluable for comparative Finno-Ugric and comparative linguistics. In fact, for anyone who already knows some Baltic, Slavic, or Germanic languages, understanding etymologies of Estonian words is a useful tool for remembering these words.
Let’s take an Estonian work raamat, meaning a ‘book’. At a first approximation, it resembles nothing. But when you look at its etymology, you realise it is related to the Latvian grāmata, meaning a ‘book’, which in turn is a borrowing from Slavic. In Russian, грамота means ‘official document’ and also ‘ability to read and write’. The Slavs borrowed the word from the Greek γράμματα ‘letters’, of which the English grammar is also a descendant.