鸡毛 literally means 'chicken feather', but in everyday speech it's often used metaphorically to refer to something trivial, insignificant, or petty — similar to 'a thing of no consequence' in English. You'll commonly hear it in the four-character idiom 鸡毛蒜皮 (chicken feathers and garlic skins), which means 'trivial matters' or 'petty details'. The metaphorical sense is the more frequent one in modern Chinese.