The word "小弟" is formed by directly combining the character "小" (xiǎo), meaning "small" or "young", with the character "弟" (dì), meaning "younger brother". This creates a compound term where "小" modifies "弟" to specify a younger or junior brother, resulting in the literal meaning "small younger brother", which is used to refer to an actual younger brother or as a humble self-reference indicating inferiority or youth.