一下下 is a colloquial measure word that means 'a tiny little while' or 'just a moment'. It's a reduplicated, more emphatic form of 一下 ('a while / a bit'), often used to sound playful or to downplay the effort involved. You can use it after a verb to indicate a very brief duration, like 'do something for just a sec'. It usually appears in spoken Mandarin and carries a light, casual tone.