split-second (n.)youdaoicibaDictYouDict[split-second 词源字典]
1884, originally the name of a type of stopwatch with two second hands that could be stopped independently. Meaning "a fraction of a second" is from 1912, from split (adj.) + second (n.1); adjectival meaning "occurring in a fraction of a second" is from 1946.[split-second etymology, split-second origin, 英语词源]