If you do something tactlessly, you do it without being careful about other people's feelings.
Tactlessly can make people feel bad or embarrassed.
Examples:
She tactlessly told him he looked funny.
He tactlessly asked about her old cat.
Tactlessly, he made fun of her drawing.
He tactlessly wore a funny hat to the party.
She tactlessly sang loudly during the quiet part of the play.
Tactlessly, she ate the last cookie without asking.