From The Collaborative International Dictionary of English v.0.48:
Lassitude \Las"si*tude\ (l[a^]s"s[i^]*t[=u]d), n. [L. lassitudo,
fr. lassus faint, weary; akin to E. late: cf. F. lassitude.
See Late.]
A condition of the body, or mind, when its voluntary
functions are performed with difficulty, and only by a strong
exertion of the will; languor; debility; weariness.
[1913 Webster]
The corporeal instruments of action being strained to a
high pitch . . . will soon feel a lassitude. --Barrow.
[1913 Webster]