In Canada "mug up" is often referred to as the last drink before the campfire is put out. With young people it is usually hot chocolate, and for older folks they might spike their coffee or hot chocolate. This term is frequently used by scouts. Winter campers are more likely to "mug up" as a means of raising their body temperature before retiring. If you're unfamiliar with the jargon, check the:

Glossary of Camping Terms