a. A lesbian
b. Off of
The correct answer is ... actually I'll let you sweat on that :)
For those not in the UK (and for those in the UK that don't know it) Offa's Dyke was a linear earthwork along the England / Wales border (http://www.offasdyke.demon.co.uk/dyke.htm) presumably to keep the barbarians out (and no, I'm not specifying which side they came from). Nowadays it is a long-distance footpath and part of our national heritage.
Ahh, happy days :)