【Droop】 , 【wilt】 , 【flag】 , 【sag】 are comparable when they mean to sink or to lose in vigor, firmness, or freshness.

【Droop】 stresses a hanging or bending downward (as through exhaustion, discouragement, or lack of nourishment).

In extended use it implies a languishing or a subsiding of something previously thriving or flourishing.

【Wilt】 applies especially to plants and suggests a loss of freshness or firmness in flower, leaves, or stems through lack of water or through excessive heat. The term often may be extended to various things that grow flaccid or weak in response to some stress (as fear, exhaustion, boredom, or a physical agent).

【Flag】 may be used of flexible things that hang loosely and limply and, with reference to plants, may be interchangeable with 【droop】 ; more often it is used of something that loses in vigor or in force so that it suggests dullness, weariness, or languor.

This effect of wearying or boring may be attributed not to the thing which drops in interest or stimulating power but to the energy, spirits, interest, or attention that are concentrated on that thing.

【Sag】 implies a sinking or subsiding, especially at one point, through undue weight, pressure, or improper distribution of stresses.

In extended uses it implies a loss of firmness, resiliency, or power to stand up against pressure, and a consequent drooping or decline.