In immediate confrontation; person to person; eyeball to eyeball, mano a mano: I go on the basketball court and have a 15-year-old guy beat me one on one/ John Rhodes was very effective, one-on-one(1967+)
: have a one-on-one with him
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with one on one
A direct encounter between two persons, especially a conflict, as in The two department heads went one on one regarding shelf space. This slangy expression almost certainly comes from sports. It is commonly used to refer to a two-person basketball game, but is also applied to the interaction of two players on opposing teams in football, soccer, and similar team sports.
[ c. 1960