of an
Meaning
(now dated) Having the same.
The two main players were roughly of an age.
(idiomatic, now dated) Indicates a more or less habitual activity during the given part of the day.
Of an evening, I like to play chess. i.e., On some evenings, I like to play chess.
Of a morning, they would work in their garden. i.e., They generally worked in their garden in the morning.
Examples
Speak no ill of a friend, nor even of an enemy.
Often an entire city has suffered because of an evil man.
It is the mark of an educated mind to be able to entertain a thought without accepting it.
The slimming of an elephant and the losses of a rich man are not noticeable.
The son of an old man is an orphan, and his wife is a widow.
Ask advice of an ignorant man and he will think you are his enemy.