wane
WAYNverbMeaning to grow smaller or weaker little by little
Memory hook WANE: a little LESS each night. Each night Pip looks out his window, and each night the moon is a little thinner. The moon wanes.
Interest in the new game waned after a few weeks.
In context The moon waxes for two weeks and then wanes, shrinking a little each night until only a thin sliver is left.
Similar decrease, fade, lessen, weaken
Opposite wax, grow, increase
From Old English wanian, “to lessen.”