The word "椅背" is formed by combining the character "椅", meaning chair, which incorporates the wood radical "木" for material and "奇" for phonetic indication, with the character "背", meaning back, which uses the flesh radical "月" for association with the body and "北" for sound. Together, they create a compound word where "椅" specifies the object and "背" denotes the part, resulting in the literal meaning "chair back".