ˈwhisperableadj. that can be whispered.Apparently an isolated use.
ΘΚΠ
the mind > mental capacity > knowledge > hiding, concealing from view > keeping from knowledge > hidden or indirect meaning > [adjective] > capable of being implied
whisperable1830
impliablea1865
1830 Blackwood's Edinb. Mag.28 893 All speak—talk—whisper—or smile, of all the speakable, talkable, whisperable, and smileable..affairs.