inaudible adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

"able to be heard," 1520s, from French audible and directly from Medieval Latin audibilis "that may be heard," from Latin audire "to hear" (from PIE root *au- "to perceive"). Related: Audibly; audibility; audibleness.

As a noun, "thing capable of being heard," from 1610s. The U.S. football sense of "signal called at the line of scrimmage to change the play called in the huddle" is by 1958.  

"state or quality of being inaudible," 1808, from inaudible + -ity.