Indisposed means unable to do something, usually because you're not feeling well or something has come up that prevents you. When your teacher is indisposed and a substitute takes over the class, it means your regular teacher couldn't make it in, often due to illness. If your mom says she's indisposed and can't come to the phone right now, she's busy with something that makes it impossible for her to talk.

The word has a polite, slightly formal quality. People rarely say “I'm indisposed” about minor things. You wouldn't say you're indisposed because you're watching TV, but you might use it if you're sick in bed or dealing with an urgent family matter. It's a tactful way of saying you can't do something without having to explain all the details.

Indisposed originally suggested being in poor health or feeling unwell, and that's still its most common meaning. But it can also mean temporarily unavailable for any reason that's nobody else's business. When someone is otherwise indisposed, they have a legitimate reason they can't participate, and the polite thing is not to press them for details.