This is the 8074th most frequent Japanese word.
定まっ
The word "定まっ" is a conjugated form of the verb "定まる," which means "to be determined," "to be fixed," or "to be settled."
The word '定まった' is used here as a past-tense verb meaning 'was decided' or 'was set', indicating that the plan's details have reached a stable decision.
計画の内容がようやく定まった。
The details of the plan have finally been decided.
In this sentence, '定まっていない' is the negative form of the verb, conveying that the destination has yet to be decided or established.
目的地はまだ定まっていない。
The destination has not been determined yet.
Here, '定まらなくなった' infers that the wind direction has lost its stability, i.e., it has suddenly become fluctuating rather than fixed or stable.