This is the 8602nd most frequent Russian word.
ононский
"Oнонский" refers to something related to the Onon River or the Onon area in Russia.
Here, 'ононской' is used as an adjective to describe the noun 'history', indicating relevance to a topic.
Он искал книги по ононской истории.
He was looking for books on Ononskaya history.
Here, 'ононский' is used to describe a type of dialect, signifying a regional linguistic attribute.
Она изучает ононский диалект для своего проекта.
She is studying the Ononsk dialect for her project.
In this sentence, 'ононских' describes 'artifacts', denoting cultural or geographic affiliation.