What are the historic definitions of the word dragon?

Solution 1:

This goes very far back in time indeed - right to the Indo-European era. There is a book that talks about the common features of Indo-European speech and poetry - still to be seen in kindred languages such as Greek, Sanskrit, Old Irish, Latin, English, etc.

It's titled How to kill a dragon, using the dragon as one theme that was common to the poetries of nations that inherited this common culture.

Of course, this doesn't directly answer your question, but this question, interesting as it is, is the topic of a book or two, like the one I recommend.

Solution 2:

Yes, the word dragon was used for living creatures in the past.

Here is the obsolete definition from OED:

A huge serpent or snake; a python. Obs. (exc. in etymol. use).

The earliest citation in OED is from c1220:

Ðe dragunes one ne stiren nout..oc daren stille in here pit.

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