Definitions of U:

• noun:   the 21st letter of the Roman alphabet
• noun:   a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
• noun:   a nitrogen-containing base found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine
• adjective:   (chiefly British) of or appropriate to the upper classes especially in language use