毛毛 is a dialectal or informal term for a baby or small child, similar to 'little one' in English. It's often used in affectionate speech (e.g., 小毛毛) and carries a cute, tender tone. This word is not the standard Mandarin 婴儿 (yīng'ér) — you'll mainly hear it in northern Chinese dialects or casual family talk. As a noun, it can be used directly or with 小 (xiǎo) as a nickname.