She was underdressed for the hiking trip and suffered hypothermia.
Examples of underdress
underdress
Her underdress is laced and plunges to daringly reveal the upper portion of her chest.
From the en.wikipedia.org
The Virgin's underdress may be visible, especially at the sleeves.
From the en.wikipedia.org
Her blue underdress is also trimmed with white fur.
From the en.wikipedia.org
Under this might be worn a linen underdress.
From the en.wikipedia.org
Shame is not a word heard often in Sin City, where few care if you underdress, overeat and drink, or ogle private parts in public.
From the usatoday.com
She has been to France three times in recent months for fittings to make sure the gown, made up of an underdress covered by a second, lace dress, was perfect for her big day.
From the mirror.co.uk
If you overdress in a casual firm you can appear dated and if you underdress in a more professional atmosphere you look like a slob and everyone assumes you're rubbish at your job.
From the express.co.uk
More examples
Dress without sufficient warmth; "She was underdressed for the hiking trip and suffered hypothermia"
Dress down: dress informally and casually; "On Fridays, employees can underdress"
(underdressed) inadequately or too informally clothed
To dress in insufficiently warm clothes; To dress in insufficiently formal clothes