If someone is pigeon-chested, the middle of the chest sticks out in a narrow, bony way, instead of looking flat or softly rounded. It is a body shape, not an illness you catch, and not a rude joke about looking like a bird — though that is where the picture comes from: a pigeon's breast is round and pushed forward. Doctors have a Latin name for a strong form of it, but ordinary English just says pigeon-chested or pigeon chest. It is a plain describing word, a little old-fashioned, more British than American. You would not shout it at someone; it can make a person feel looked at.