Pluck something out of the air
Definition 1 of 1
Pro Tip 1/3Guesswork Implication
The phrase suggests the speaker is guessing and is unsure about the exact details. 
He plucked an excuse out of the air to avoid going to the party.
Pro Tip 2/3Spontaneity
Use it when someone makes a spontaneous or sudden decision without much thought. 
She plucked a destination out of the air for her random weekend getaway.
Pro Tip 3/3Lack of Evidence
It implies that there isn't substantial evidence or reasoning supporting the statement. 
The accused plucked an alibi out of the air, which didn't hold up in court.