A doddle is something that is very easy to do. In ordinary English you almost never hear it as a verb, though "to be very easy" is the idea: people say "It's a doddle" or "That exam was a doddle." It is informal British English. Americans are more likely to say "a piece of cake" or "a breeze." You use it about a task, a test, a form, a job — not about a person. If you call a person a doddle, it will sound odd. Teachers may use it to calm a class ("the first question is a doddle"), and friends use it to boast a little. The origin is not certain. It possibly comes from a dialect word doddle, "to toddle, to sway, to nod drowsily" — the picture being that the job is so easy you could do it while wandering along. Treat that as a "possibly," not a fact.

小菜一碟、輕而易舉的事（英式口語；美式較常說 a piece of cake）