underdressed
adjective uk/ˌʌn.dəˈdrest/ us/ˌʌn.dɚˈdrest/
穿着不够漂亮（或正式）的
wearing clothes that are not attractive enough or formal enough for a particular occasion
Everyone else was in formal suits and I felt completely underdressed. 其他人个个都西装笔挺，我感觉自己穿得有点儿太随便了。