Wiktionary
vb. 1 (context idiomatic English) To relax; to make quieter or less obtrusive; to make milder. 2 To moderate or relax; to diminish or weaken the striking characteristics of; to soften. 3 To make a television program, piece of writing, etc. less offensive and so more suitable for a family audience.
WordNet
Usage examples of "tone down".
Still, he made a mental note to suggest that she might want to tone down the inside info in her next letter.
Whether this is because they've heard some ugly rumours or just because on this day they are required to tone down the celebrating, I don't know, but even Irith Victorious looks subdued.
He hustled his step to meet Corsi, hoping his prox-imity might prompt her to tone down.
He was all fire and air, it seemed, and could not be bothered to tone down his dithyramb.
Tall and gaunt, with tendons and veins rising from his pale skin, Wallace tried to tone down his intimidating physique and face with brown tweed jackets, corduroy slacks, and pastel shirts, an attempt that was, as today, completely unsuccessful.