i-adjective
uncertain; transient; changeable
Describes something that cannot be fixed or relied upon; often used for life, the world, or fate. Literary and somewhat old-fashioned.
世の中は定めないものだ。
The world is an uncertain place.
人の心は定めない。
People's hearts are changeable.
定める is the transitive verb meaning 'to decide' or 'to establish'; 定めない is its negative form used as an adjective meaning 'not decided' → 'uncertain'.
不確か is a more common and neutral word for 'uncertain'; 定めない is more literary and emphasizes the inability to be fixed.
Negative form of the verb 定める (to decide, to establish), used adjectivally. The kanji spelling 定め無い explicitly marks the negative with 無い.