Weathercock
An inconstant person, someone who changes (decisions/opinions/plans) with the wind.
"Jacob is such a weathercock, he said wanted to go shopping with Dari today, but he changed his mind like one minute later."
weathercock logic
the fallacious logic that justifies an action or result in whatever way it occurs. If it turns out to be good, it is justified and if not, it is also justified. For example, if a student fails his exams, this logic states that he fails because he has not studied hard. If he passes, it defends him by stating that he passes because of his diligence.
A weathercock logic is not reliable.