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. upside-down cake a sponge cake baked with fruit in a syrup at the bottom, and inverted for serving.
[ ME, orig. up so down, perh. = 'up as if down' ]