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insure: insure: [15] Insure and ensure [14] are ultimately the same word. And their common ancestor started out, in fact, as a variant of assure [14]. This came via Old French asseurer from Vulgar Latin *assēcūrāre, a compound verb formed from the Latin prefix ad- ‘to’ and the adjective sēcūrus ‘safe’ (source of English secure and sure).

Anglo-Norman had a variant form, enseurer, which produced English ensure. From fairly early on this had been alternatively spelled insure (using the Latinate prefix in-), but it was not until the 17th century that this version became established in the sense ‘provide cover against loss, damage, etc’ (for which previously the more usual term had actually been assure).
=> assure, ensure, secure, sure
insurgent: insurgent: [18] An insurgent is etymologically someone who ‘rises up’. The word comes from the Latin verb insurgere, which was formed from in- in the sense ‘against’ and surgere ‘rise’ (source of English surge and source). An insurgent is hence fairly straightforwardly a rebel, someone taking part in an uprising, a belligerent who is not part of an officially recognized fighting force.

Choice of vocabulary in this area tends to be controversial, however, and the use of insurgent to denote Iraqi irredentists after the Coalition invasion of 2003 was widely criticized – partly, perhaps, from the misconception that they were being characterized as ‘surging in’ from outside the country. The longer established insurrection [15], from the same ultimate source, is much less liable to such misunderstanding.
=> resource, resurrection, source, surge
counterinsurgency (n.): 1962, from counter- + insurgency.
insurance (n.): 1550s, "engagement to marry," a variant of ensurance (see ensure). Commercial sense of "security against loss or death in exchange for payment" is from 1650s. Assurance was the older term for this (late 16c.).
insure (v.): mid-15c., insuren, spelling variant of ensuren (see ensure). Took on its particular sense of "make safe against loss by payment of premiums" from mid-17c. (replacing assure in that meaning). Related: Insured; insuring.
insurer (n.): 1650s, agent noun from insure.
insurgence (n.): 1847; see insurgency + -ence.
insurgency (n.): 1803, from insurgent + -cy.
insurgent (n.): "one who rises in revolt," 1765, from Latin insurgentem (nominative insurgens), present participle of insurgere "rise up, rise against, revolt," from in- "against," or perhaps merely intensive, + surgere "to rise" (see surge). An obsolete verb insurge "to rise in opposition or insurrection" is attested from 1530s.
insurmountable (adj.): 1690s, from in- (1) "not, opposite of" + surmountable. Related: Insurmountably. Brachet calls French insurmontable a "ghastly philological monster."
insurrection (n.): early 15c., from Middle French insurrection, from Late Latin insurrectionem (nominative insurrectio) "a rising up," noun of action from past participle stem of insurgere "to rise up" (see insurgent).
insurrectionary: 1796 (adj.), 1893 (n.), from insurrection + -ary.