Tacitly is the adverb of tacit: doing something without saying it, in a way that everyone still understands. It attaches to verbs of agreeing, allowing, accepting, admitting, and supporting — you tacitly agree, tacitly accept, tacitly admit, tacitly approve, tacitly acknowledge.

The useful thing about the adverb is that it lets you say how an act was done rather than describing the agreement afterwards. 'They tacitly accepted the new rules' tells you nobody ever announced acceptance; they simply started following the rules, and that was answer enough. The action itself, or sometimes the lack of any action, does the speaking.

A very common pattern is 'by doing X, they tacitly did Y' — by staying, by paying, by saying nothing, by continuing as before. Silence is the most frequent way of tacitly doing anything, which is why 'tacitly admitted' often carries a slight sting: the person did not confess, but what they did left no other reading.

Same register as tacit — formal, written, common in news reports, legal writing and academic work. In everyday conversation Taiwanese speakers of English are far more likely to hear 'they didn't say anything, but everyone knew'.