adv. & adj.
--adv.
1. with the upper part where the lower part should be; in an inverted position.
2 in or into total disorder (everything was turned upside down).
--adj. (also upside-down attrib.) that is positioned upside down; inverted.
Phrases and idioms:
upside-down cake a sponge cake baked with fruit in a syrup at the bottom, and inverted for serving.
Etymology: ME, orig. up so down, perh. 'up as if down'