Lewis & Short

sĭlentĭōsus, a, um, adj. [silentium], perfectly still or silent: nactus opacae noctis silentiosa secreta, App. M. 11 init.
Adv.:
sĭlentĭōsē, stilly, silently: silentiose geritur publicum bonum, Cassiod. Var. 11, 1 med.