Is it ensure, insure, or assure?
1931, "mountaineering technique for descending steep faces," from French rappel, literally "recall" (Old French rapel), from rapeler "to recall, summon" (see repeal (v.)). The same word had been borrowed earlier (1848) to mean "a drum roll to summon soldiers."
1957 in the mountaineering sense; see rappel (n.). Related: Rappeled; rappelling.