Upside down means with the top part at the bottom. Inverted, and usually by accident.
It works as an adverb and, hyphenated, as an adjective, so a jar stored upside down and an upside-down cake. There is a figurative sense about disorder, since turning a room upside down means searching it thoroughly and turning somebody's life upside down means disrupting it completely. Australia is affectionately called the land down under rather than upside down, which is a distinction Australians are firm about.
1phraseupside downWith the upper part at the bottom.
“The picture had been hung upside down.”
2phrasecompleteIn complete disorder.
“They turned the whole flat upside down looking for it.”
Upside down was up so down. The Middle English phrase was up-so-doun, with so in the middle, and the side we now say is a later reshaping of that little word.
Middle English
up sid doun
English, today
upside down
Built from
From Middle English up sid doun, an alteration of earlier upsadowne, upsedowne, up-so-doun (“upside down”), equivalent to up + so + down.
Etymology from Wiktionary, CC BY-SA