Webster's Revised Unabridged Dictionary · 1913

Unwell

Unwell , adjective

1.
Not well; indisposed; not in good health; somewhat ill; ailing.
2.
(Medicine) Specifically, ill from menstruation; affected with, or having, catamenial; menstruant.

This word was formerly regarded as an Americanism, but is now in common use among all who speak the English language.