From 浮(う)き (uki, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 浮(う)く (uku).”) + 袋(ふくろ) (fukuro). The fukuro changes to bukuro as an instance of rendaku (連濁).
うきぶくろ • (ukibukuro) ←うきぶくろ (ukibukuro)?