"underdressed" in the verb sense
1. underdress
dress without sufficient warmth
"She was underdressed for the hiking trip and suffered hypothermia"
2. dress down, underdress
dress informally and casually
"On Fridays, employees can underdress"
"underdressed" in the adjective sense
1. underdressed
inadequately or too informally clothed
Source: WordNet® (An amazing lexical database of English)
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