The Glastonbury festival is a huge music festival taking place on a farm during the summer in the UK, it is notorious for getting rained on, and the whole festival ground becoming waterlogged and muddy.

So the Glastonbury tan is where the festival goers get brown all over due to being caked in mud.
my brother came home from the festival with a Glastonbury Tan
by junkothegreat June 30, 2009
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