The word "一丝一毫" is formed by juxtaposing two parallel phrases: "一丝" (meaning "one thread") and "一毫" (meaning "one hair"). Each phrase consists of the numeral "一" (one) followed by a noun representing a minute unit ("丝" for silk thread and "毫" for fine hair), with both units symbolizing extreme smallness. The repetition of the structure reinforces the meaning, creating a compound that denotes an insignificantly tiny amount through direct lexical pairing.