"vainest" in the adjective sense
1. conceited, egotistic, egotistical, self-conceited, swollen, swollen-headed, vain
characteristic of false pride having an exaggerated sense of self-importance
"a conceited fool"
"an attitude of self-conceited arrogance"
"an egotistical disregard of others"
"so swollen by victory that he was unfit for normal duty"
"growing ever more swollen-headed and arbitrary"
"vain about her clothes"
2. bootless, fruitless, futile, sleeveless, vain
unproductive of success
"a fruitless search"
"futile years after her artistic peak"
"a sleeveless errand"
"a vain attempt"
Source: WordNet® (An amazing lexical database of English)
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