The new sofa looks nice, but it's actually quite uncomfortable to sit on.

uncomfortable 是由前缀 un-（表示"不、非"）和形容词 comfortable（舒适的）组成的复合词。comfortable 来自动词 comfort（使舒适）加后缀 -able（能够...的）。uncomfortable 整体意思是"不舒服的"。