Stands for Barely Legal Geoff, indicating a Geoff of questionable age.
Frikipedia advises leaving well alone.
Lots of BLGs can be found in Worcester, particularly at Bamboo. Again, leave well alone. Frikipedia researchers discovered that after drinking lots of WKD Blues in an attempt to fit in with the younger crowd, engaging in conversation with BLGs is impossible anyway. In fact, doing anything but running round the beer garden in ever-decreasing circles is impossible.