The term for fickle
means reckless, undisciplined
(see also Judges 9:4). Here it likely refers to the way these prophets gave no thought to the consequences of their actions of falsely claiming to be speaking in the name of the Lord. Zephaniah accused the prophets of neglecting the charge and call of God in their lives. The very thing they were supposed to do, they failed to do.
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4 Her prophets are fickle, treacherous men; her priests profane what is holy; they do violence to the law.