When i was at school i always remember a history teacher telling us that the word 'fortnight' dates from Norman times. When a castle or fort would be sieged, the enemy would surround it, and those persons locked inside would have to fend off the aggressors until all supplies of food and ammunition ran out. This could take several weeks, if not months to achieve. Therefore, it became barrack-humour that anyone sent to defend a stronghold would be off to 'spend a fort-night'.
I don't know if there's any truth in this, but i love the story!