Insufficiently dressed, as for cold weather.(adjective)
Insufficiently well-dressed, as for a formal event.(adjective)
Simple past tense and past participle of underdress.(verb)
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Use "underdressed" in a sentence
"For some of their fresh-faced interns, however, skimpy tank tops, jeans, short skirts and flip-flops are the "underdressed" norm."
"And the announcement was out that Robert Heinlein wanted SFWA members to really dress for the awards, I saw people in tuxes, and I knew for one event I must surely be underdressed."
"Or maybe those women were riduculously underdressed?"