The Collaborative International Dictionary of English v.0.48:

Upside \Up"side`\, n.
   1. The upper side; the part that is uppermost.
      [1913 Webster]

   2. the benefits; the positive features; -- said of a
      situation or event that has both positive (good) and
      negative (bad) aspects.
      [PJC]

   To be upsides with, to be even with. [Prov. Eng. & Scot.]
      --Sir W. Scott. --T. Hughes.

   Upside down. [Perhaps a corruption of OE. up so down,
      literally, up as down.] With the upper part undermost;
      hence, in confusion; in complete disorder; topsy-turvy.
      --Shak.
      [1913 Webster]

            These that have turned the world upside down are
            come hither also.                     --Acts xvii.
                                                  6.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Upside down \Up"side down`\, adv.
   in such a manner that the part normally pointed upward is
   pointed downward; same as upsidown and upsodown.
   [PJC]

WordNet (r) 3.0 (2006):

upside down
    adv 1: in an inverted manner; "the box was lying on the floor
           upside down"