say-so vs dictum

say-so and dictum both are nouns.

Word	Noun	Adjective	Verb	Adverb
say-so	Yes	No	No	No
dictum	Yes	No	No	No

As nouns, say-so and dictum are synonyms defined as:

Other synonyms of say-so include pronouncement.

say-so (noun)	dictum (noun)
the power or right to give orders or make decisions	an authoritative declaration
an authoritative declaration	an opinion voiced by a judge on a point of law not directly bearing on the case in question and therefore not binding
one chap's arbitrary assertion