down-low
noun [ S ] mainly US informal uk/ˈdaʊn.ləʊ/ /daʊnˈləʊ/ us/daʊnˈloʊ/ /ˈdaʊn.loʊ/
on the down-low 秘密地;悄悄地
done secretly and not discussed, so that only a small number of people know about it
Two colleagues were carrying on a relationship on the down-low. 两位同事在低调地谈恋爱。
Let's keep it on the down-low, okay? 我们保持低调，好吗？