Despite the bad weather, he looked completely unworried and kept enjoying his picnic.

unworried 是由前缀 un-（表示"不、非"）和形容词 worried（担心的）组成的形容词。un- 表示否定，worried 来自动词 worry（担心），所以 unworried 意思是"不担心的、无忧虑的"。