There is a classic Tom Swifty joke - I don't know who thought of it first - that runs as follows

"You must be my host," Tom guessed.

This is rather neat, especially given host and guest turn out to closely related words. In the reconstructed Proto-Indo-European tongue believed to be the distant ancestor of many languages alive today they were the same word, which meant both host and stranger/guest, and was something like ghosti.

This ancestor language predated the invention of writing, so this is just something we have worked out from the evidence of more recent languages that were written down (for example Latin and Old English etc.), and of course the languages spoken at the present time. However scholars are fairly confident our reconstruction is broadly right. 

The original word has come down to us through different routes and so host and guest have separated in English, but in some languages the equivalent word seems to retain the dual sense; for example Italian ospite.

I suppose we could say this is a contronym (see here).