'one-on-one' originates from English, specifically the phrase 'one on one', where 'one' meant the numeral 1 and 'on' was used in the sense of 'against' or 'opposite'.

'one on one' appears as a descriptive phrase in sporting and informal contexts (early 20th century) to mean one person versus another; over time the phrase was commonly used for private meetings and was often written with hyphens as 'one-on-one' in modern English.

Initially, it often referred to direct athletic contests ('one person against another'); over time the meaning broadened to include private or individual meetings and interactions.