One possible opposite of "arbitrary" is "principled."
random, chance, subjective, inconsistent, erratic, personal, whimsical, capricious
Haphazard and chance are good ones.
If you don't like that, maybe obscure is also a good one.
A synonym for fortuitous is random.
random, variable, inconstant, unforseeable
synonymize
"Alternate word" is a noun-phrase that is kind of a synonym for the word "synonym. There is no one exact word that means the same as synonym.
The antonym of the word 'synonym' is 'antonym'.
A synonym for fortuitous is random.
unusual
Incorrect, random
Another synonym is intermittent. You could also use the words random irregular occasional
random, variable, inconstant, unforseeable
synonymize
"Word for word" is a synonym for verbatim.
"Alternate word" is a noun-phrase that is kind of a synonym for the word "synonym. There is no one exact word that means the same as synonym.
The antonym of the word 'synonym' is 'antonym'.
"Alternate word" is a noun-phrase that is kind of a synonym for the word "synonym. There is no one exact word that means the same as synonym.
accidental, chance, by chance, random, incidental
what is a synonym for the word felony