A contraption is a machine or a bit of equipment, but the word already smiles at it. It looks homemade, odd, or too clever — as if it might not really work, or might work in a funny way. You would not call a new hospital scanner a contraption unless you meant to be rude. You might call a student’s wild homework robot a contraption, with a kind laugh. Informal, and common in American English; British English uses it too, with the same slightly comic feel. Though actually it does not have to be broken. It only has to look unlikely.