1860-65; < French: literally, lightning (flash), Old Frenchesclair, noun derivative of esclairier to light, flash < Vulgar Latin*exclariāre, for Latinexclārāre, equivalent to ex-ex-1 + clārāre to make bright, derivative of clārusclear
1861, from Fr. éclair, lit. "lightning," from O.Fr. esclair, from esclairare "to light up, make shine," ult. from L. exclarare "light up, illumine," from ex- "out" + clarus "clear." The sense is perhaps the "lightness" of the confection.