tandemyoudaoicibaDictYouDict
tandem: [18] Latin tandem meant ‘eventually, at length’. Its use for ‘acting conjointly’ arose from an 18th-century play on words, in which ‘at length’ was jocularly interpreted as ‘lengthwise, in a straight line’, and the word was applied to a ‘carriage drawn by two horses one behind the other in a straight line’. In the 1880s it was transferred to a ‘bicycle with two seats, one behind the other’. Its more general modern use, for ‘acting together’, dates from the early 20th century.
tandem (n.)youdaoicibaDictYouDict
1785, "carriage pulled by horses harnessed one behind the other" (instead of side-by-side), jocular use of Latin tandem "at length (of time), at last, so much," from tam "so" (from PIE *tam-, adverbial form of demonstrative pronoun root *-to-; see -th (1)) + demonstrative suffix -dem. "Probably first in university use" [Century Dictionary]. Transferred by 1884 to bicycles with two seats. In English as an adverb from 1795; as an adjective from 1801.