Sudden, overwhelming love at first sight.
Sudden, overwhelming love at first sight — a “bolt of lightning.” (Also literally a lightning strike, and figuratively any sudden, unforeseen event.)
The instantaneous, jolting onset of love, emphasizing suddenness and electric intensity. It differs from limerence (a prolonged obsessive state), infatuation (which implies shallowness), and kilig (a giddy thrill, not necessarily love); and from koi no yokan, which is the premonition of love to come, not the strike itself. Maps closely to English “love at first sight,” foregrounding the violent suddenness.
coup “blow, strike” + de “of” + foudre “lightning” = “strike of lightning.”
Literally “stroke of lightning”: coup (from Latin colaphus “a blow,” via Greek kolaphos) + foudre “lightning.” Borrowed into English by 1779 meaning “a sudden, unforeseen occurrence.”
The lightning-strike expression first denoted a sudden, surprising (often unpleasant) event; the “love at first sight” sense emerged via lightning as a metaphor for sudden attraction. It remains both a live literal term and a romantic idiom.
Not generally claimed “untranslatable” — “love at first sight” is an accepted equivalent. The nuance to flag is that it is both a literal term (lightning) and an idiom, the romantic sense being a metaphorical extension.