Ugg boots are a unisex style of sheepskin boot originating in Australia and New Zealand. The boots are typically made of twin-faced sheepskin with fleece on the inside, a tanned outer surface and a synthetic sole. The term "ugg boots" originated in Australia, initially for utilitarian footwear worn for warmth, and which were often worn by surfers during the 1960s. In the 1970s, the boots were introduced to the surf culture of the United Kingdom and the United States. Sheepskin boots became a fashion trend in the U.S. in the late 1990s and as a worldwide trend in the mid-2000s. In Australia, they are worn predominantly as slippers and often associated with "daggy" fashion sense and "bogan" culture.