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.
His answer was a fixed, predictable one.
I got tired of the same old routine.
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.
Standard spelling. The kanji 決 conveys 'decided' or 'fixed'.
Variant with full kanji for 切る. Less common but still used.
Rare variant omitting the okurigana ま.
Rare variant using 極 (extreme) instead of 決. Not standard.
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'.