WordNet sense 1 (of relatively small extent from one surface to the opposite or in cross section):

thin

WordNet sense 2 (lower than previously):

down

WordNet sense 3 (less than normal in degree or intensity or amount):

low

WordNet sense 4 (low in spirits):

blue, dispirited, downcast, downhearted, low-spirited, down in the mouth

WordNet sense 5 (affected or marked by low spirits):

dejected

WordNet sense 6 (having the central portion lower than the margin):

indented

WordNet sense 7 (curving inward):

concave