- Lassitude is defined as a feeling of lethargy or a lack of mental or physical energy.
When you are in a state where you don't want to do anything except lay around, this is an example of lassitude.
lassitude definition by Webster's New World
Origin: French ; from Classical Latin lassitudo ; from lassus, faint, weary: see late
lassitude definition by American Heritage Dictionary
Origin: Middle English, from Old French, from Latin lassitūdō, from lassus, weary; see lē- in Indo-European roots.
lassitude - Medical Definition