Old English stille "motionless, stable, fixed, stationary," from Proto-Germanic *stilli- (source also of Old Frisian, Middle Low German, Middle Dutch stille, Dutch stil, Old High German stilli, German still), from PIE *stel-ni-, suffixed form of root *stel- "to put, stand, put in order," with derivatives referring to a standing object or place. Meaning "quiet, calm, gentle, silent" emerged in later Old English. Euphemistic for "dead" in stillborn, etc. Still small voice is from KJV:
And he said, Go forth, and stand upon the mount before the LORD. And, behold, the LORD passed by, and a great and strong wind rent the mountains, and brake in pieces the rocks before the LORD; but the LORD was not in the wind: and after the wind an earthquake; but the LORD was not in the earthquake: and after the earthquake a fire; but the LORD was not in the fire: and after the fire a still small voice. [I Kings xix.11-13]
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).