A deliberate Latin pun — 'tandem' (at last, finally) was humorously misused to mean 'lengthwise' by English university students.
An arrangement of two things one behind the other; a bicycle built for two riders.
From Latin 'tandem,' which actually means 'at length, finally' (in time). English university students made a pun: they used the temporal 'at length' as if it meant spatial 'lengthwise' — horses arranged 'at length' (one behind the other) instead of 'at length' (eventually). The joke stuck. Key roots: tandem (Latin: "at length (in time), at last").
'Tandem' is based on a Latin pun that nobody finds funny. Latin 'tandem' means 'at last, finally' — it's about time, not space. Some wit at an 18th-century English university deliberately misused it to mean 'lengthwise' (one after another in space), probably to show off that they knew enough Latin to make a terrible joke about it. The pun was so bad it became permanent. Every tandem bicycle is riding on an 18th-century dad joke