noun-modifying (irregular)
fixed; obvious; commonplace; stereotyped
Describes something that is predetermined, lacking originality, or so typical as to be unremarkable. Often carries a slightly negative nuance of being clichéd or boring.
彼の答えは決まりきったものだった。
His answer was a fixed, predictable one.
決まりきった日常に飽きてしまった。
I got tired of the same old routine.
The base verb meaning 'to be decided' or 'to be fixed'. 決まりきった is the past tense form used adjectivally, emphasizing that something is already completely decided or stereotyped.
Also means 'commonplace' or 'hackneyed', but ありきたり is a na-adjective/noun that focuses on lack of novelty, while 決まりきった emphasizes being predetermined or fixed.
From the verb 決まる (to be decided) combined with 切る (to cut, to do completely) in the past tense, literally 'has been completely decided'. Used as a pre-noun adjectival to mean 'fixed' or 'stereotyped'.