vainly

Vainly

The word "vain," from which "vainly" comes from, has a couple of meanings that are very different from each other. It refers to showing overconfidence in physical looks or accomplishments, doing something for hopeless reasons, or to something that is unnecessary, etc. In this line, 2 of the meanings seem to have been employed. One way that the word is used is when the speaker means that he hopelessly believes that his lover thinks that he is young. He regards it as a hopeless effort because it is not true that he is young and since he can see through her lies of infidelity, then there is a high chance of her being able to see through his lies. The other meaning of "vainly" that contributes to the line is to show overconfidence in physical looks or accomplishments. This is used because he also allows himself vanity to think that he could physically pass for a man in his youth.