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Being 'a good sport' means to have a good attitude, to take it well when something doesn't go your way. For example, in an actual sports context - let's say a runner falls down mid race, and a competitor stops to help them up and finish the race. We could say 's/he is such a good sport'.
Outside of a sports context, we use it for someone who takes jokes well, or goes along with the situation. For example, 'We made jokes at Mike's expense all night, but he's a good sport' (i.e he always takes it well and doesn't get upset).
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