How does one explain 'the arbitratiness of the sign' as one of Saussurean's principle?

Saussure's principle regarding "the arbitrariness of the
sign" refer to how meanings of words are given purely arbitrarly compared to the
physicality of the word. What that means is that words are "unmotivated signs" - there
is no inherent connection between a word and what it connotates or denotates. For
example, the word "tree" is in no way 'appropriate' to what it means. If one person says
"tree" to three people who are listening, one person may think of a 'maple tree,'
another a 'pine tree,' and another an 'apple tree.' There is no definition for a word
because of how the word looks. (Minor exceptions may be onomatopoeia, but it is
important to bear in mind that these vary by language.)