What the upside down Y is
The capital upside down Y ⅄ is officially the turned sans-serif capital Y (U+2144), from the Letterlike Symbols block. The lowercase ʎ is the Latin small letter turned Y (U+028E), from the IPA Extensions block. Both are real letters in their own right, which is why they paste correctly almost everywhere.
In the International Phonetic Alphabet, ʎ stands for the palatal lateral sound, like the "gl" in Italian figlio or the traditional pronunciation of Spanish "ll". Upside-down text generators also use ⅄ and ʎ as the flipped Y, which is how most people come across them.
Look-alikes and what they mean
Several characters look like an upside down Y but mean something different. λ is the Greek letter lambda, used in physics for wavelength and in programming for anonymous functions (see the lambda symbol page). ∧ is the logical AND sign, and ⋏ its curly version. Ʌ and ʌ are the turned V, which is the IPA symbol for the vowel in "cup".
人 is a Chinese character meaning "person", also used in Japanese. ᛦ is the runic letter yr, which looks like the rune ᛉ (algiz) turned upside down. Pick the one that matches your purpose: ⅄ or ʎ for flipped text, ∧ for logic, λ for Greek and math.
How to type ⅄ and ʎ
There is no Alt code for these letters. In Microsoft Word, type 2144 and press Alt + X for ⅄, or 028E for ʎ. In HTML, use ⅄ for ⅄ and ʎ for ʎ. On a Mac, open the Character Viewer with Control + Command + Space and search "turned y".
To flip a whole word or sentence, use the upside down text generator. It also reverses the letter order, so "YES" becomes SƎ⅄. More flipped and fancy alphabets are on the letter symbols page.