The word may have derived from fug boots that were worn by aviators in rural Australia during World War I. The term is believed to be a shortened version of “flying ugg boots.”

The unisex sheepskin and fleece footware called UGG boots, or simply uggs, gained popularity in the 1960s when competitive surfers began wearing them. They were the perfect remedy for cold, numb, wet feet.
Now, everyone seems to be sporting the boots, from Hollywood celebrities and their tween followers to suburban moms. Originally of simple design, Uggs now sport glamorous embellishments such as brass grommets, fringe, and animal skin patterns.

So, what does the word mean?
The history of the term ― and the trademark — are highly disputed. Australia and New Zealand both claim to be the original home of Uggs and in these countries the term “ugg” can be used to market any fleece and sheepskin boot. That is, it is considered a generic term. However, in more than 100 other countries, UGG is a registered trademark.