It's due, Lap

I mentioned to my French PF friend who had emailed me shortly after my partner and I had just been talking about him, that his ears must have been burning.

It's an old term I've heard and used many times. What are it's origins? Why would one's ears be burning if one is being discussed in their absence?

Recall, if you can, when an embarassing thing happened to you and - forfend it not - you blushed!

When blushing, ears go very pink and feel sensitive - hence the 'burning' form inside and out.