One of the most powerful forces in the world's weather is named after a baby. El Niño means "the child" in Spanish, and the child in question is Jesus.
The name started as local knowledge. Sailors at Paita, a port in northern Peru, noticed a warm current that flowed south along the coast. It tended to appear just after Christmas, so they called it the current of El Niño, the Christ child.
The first scientific record came in 1891. Dr Luis Carranza, president of the Lima Geographical Society, wrote a short note in the society's bulletin. He described a north-to-south countercurrent between Paita and Pacasmayo and passed on the sailors' name for it. In 1895 Federico Alfonso Pezet brought the subject to the Sixth International Geographical Congress in London.
English picked up the term quickly. Merriam-Webster gives 1896 as its first known use, and its etymology reads "Spanish, literally, the child (i.e., the Christ child)." The dictionary explains that the flow tended to appear at the Christmas season.
For decades the name meant a regional current. Its scope grew in the 1960s, when scientists widely realized that the Peruvian warming was tied to changes across the tropical Pacific. In 1969 Jacob Bjerknes of UCLA gave the first description of the physical mechanism behind El Niño and a linked pattern called the Southern Oscillation.
The combined system is now called ENSO, for El Niño/Southern Oscillation. El Niño is its warm phase, when surface waters in the central and eastern tropical Pacific run warmer than usual. According to NOAA, El Niño recurs every two to seven years on average.
The cold phase needed a name of its own. Princeton's S. George Philander is credited with coining La Niña, "the girl," for the opposite end of the cycle.
So a Christmas nickname from a fishing coast became a scientific term used around the world. Every forecast that mentions El Niño still carries the sailors' reference to the holiday.