instability n.

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.

mid-12c., "trustworthy, reliable;" mid-13c., "constant, steadfast; virtuous;" from Old French stable, estable "constant, steadfast, unchanging," from Latin stabilis "firm, steadfast, stable, fixed," figuratively "durable, unwavering," literally "able to stand," from PIE *stedhli-, suffixed form of root *sta- "to stand, make or be firm." From c. 1300 as "well-founded, well-established, secure" (of governments, etc.). Physical sense of "secure against falling" is recorded from late 14c.; also "of even temperament." Of nuclear isotopes, from 1904.