From 受(う)け (uke, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 受(う)ける (ukeru).”) + 箱(はこ) (hako). The hako changes to bako as an instance of rendaku (連濁).
うけばこ • (ukebako) ←うけばこ (ukebako)?