Saussure says that the nature of the relationship between the constituents of the sign, the signifier and the signified is arbitrary. Consider what Saussure means by this “arbitrary” nature of the sign.
What Saussure is getting at when he uses the term “arbitrary” is that the word we use to signify, say “car” is, is just the sound wave we use to represent that personal vehicle in my driveway. The word “car” isn’t actually The Car Itself, it’s just an agreed upon soundwave we use to signify The Car Itself.
Of course we have to keep in mind that this word, “car”, is something we are all agreeing to use so in that sense it is not completely arbitrary since both you and I have been taught this word and agree to continue to use it. Saussure says the words we are using are not “left entirely to the speaker,” but rather the word “has no natural connection with the signified,” (79).
However, there is nothing to stop us from agreeing to use a different word, such as in the 1985 episode of The Twilight Zone, “Wordplay” where society decides to change what every word means, much to the frustration of the main character who never got the memo and winds up having no idea how to speak the new language.
Also when confronted with something that as yet does not have a name, while a new word to signify this new object can be picked quite arbitrarily (such as the infamous “Boaty McBoatface”), the person or people giving the name often draw upon established language as guidance, such as using Latin to name a new biological species. In such a case there is a naming convention agreed upon to deal with new discoveries in order to maintain consistency rather than being completely arbitrary.
Saussure goes on to explore onomatopoeia and interjections as being possible exceptions to the rule, but even in these cases we see in comparing languages there is a large degree of variance: in English, when we hurt ourselves we say “ouch”, in French it’s “aie”, and when a dog barks we mimic it in English by saying “bow-wow”, whereas in French “oua-oua” is used. These examples sound quite different, even though they are supposedly mimicking the typical sound of a dog’s bark.