蔫 has two parallel meanings: literally for plants/leaves losing water and going limp, and figuratively for people lacking energy or enthusiasm. Common compounds:
蔫巴巴 (niānbābā, 'all wilted/listless'),
蔫头耷脑 (niāntóu-dānǎo, 'head drooping, listless'),
蔫不唧儿 (niānbujīr, Beijing slang: 'quietly / on the sly').