A one legger is a term used in the world of sales. They refer to a sales appointment, meeting, or pitch, where one of two or more decision-makers are present. The present individual, of course, cannot proceed with the sale without consulting the other decision-maker (the other leg) first.
A sales appointment in which the salesperson shows up to a client's house, only to find one spouse not home. The sale is not completed as the present spouse (the one legger) must talk it over with the non-available spouse.