A crackpot is a colorfully strange or odd person. You might describe your neighbor as a crackpot if he keeps farm animals inside his house and wears a clown costume everywhere he goes.
Although you might call someone a crackpot if he's eccentric or has extreme, unusual beliefs or opinions, the word is fairly derogatory and might offend him. People who are called crackpots often take up unpopular causes or conspiracy theories. A scientist who doesn't believe in the theories of evolution and gravity could be called a crackpot. Dating from the late 19th century, crackpot combines pot, slang for "head," with crack, implying someone whose head is cracked.