Good-naturedly is an adverb that describes someone who is kind, friendly, and easygoing. However, if you want to express the opposite of good-naturedly, then you have a few antonyms to choose from. For example, an antonym for good-naturedly could be harshly, which describes someone who is unkind, rude or severe in their interactions with others. Another antonym is negatively, which displays pessimism, distrust or ill-will. Unpleasantly is another antonym for good-naturedly, which conveys someone who is unpleasant or disagreeable in their behavior. Overall, antonyms for good-naturedly represent individuals who exhibit traits that are the opposite of kindness, easiness, and friendliness.