pluck something out of the air
pluck (something) out of the/thin air
To say or produce something haphazardly or at random, rather than through careful consideration or calculation. Look, this legislation hasn't been plucked out of thin air. We've spent months crafting the bill to best serve all of our citizens. Her ability to ad lib is so remarkable that it's hard to believe she's plucking these jokes out of the air.