Not just boots, but a brand of shoes specializing in boots.
They do not always have sheepskin insides and suede outsides, although most do. Most people have a strong opinion about UGGs, either favoring them or despising them.
The only defense that UGG-haters have is that they're probably called UGG because they make ugly boots, which is the most unoriginal pun ever spat out by the Tard Machine.
Idiot: Ew! Why are you wearing UGGs? I HATE them!
UGG wearer: Are you saying I should dress according to your likes and dislikes?
Idiot: Well, they're ugly. That's why they're called UGGs!
UGG wearer: I'm pretty sure a marketing team wouldn't name their shoes UGGs because they want to point out they make ugly boots. I mean, wouldn't their sales drop?