The word "叔叔" is formed through the process of reduplication, where the single character "叔" (which historically denotes a younger brother of one's father) is doubled. This repetition serves to create a more familiar, affectionate, and commonly used term of direct address. The duplicated form softens the formality of the standalone kinship term, transforming it into the warm, everyday word used by children and adults to call or refer to a paternal uncle or, by respectful extension, an unrelated older man.