The notion that words are arbitrary and have no meaning in themselves refers to which characteristic of language

Language is Symbolic. This characteristic holds that words have no meaning in themselves and are therefore, arbitrary.

Language is primarily comprised of sound symbols, with their graphological counterparts, and is used to refer to objects, occurrences, or to give meaning. The meaning of these symbols were initially arbitrarily assigned and chosen, and then became conventionally accepted and employed. The words in a language are therefore "symbols of meaning."