on a first-name basis

*on a first-name basis (with someone)

knowing someone very well; good friends with someone. (*Typically: be ~; get ~.) I'm on a first-name basis with John. John and I are on a first-name basis.
See also: basis, on

on a first-name basis

Quite familiar and friendly, as in Practically all the guests were on a first-name basis. This idiom alludes to the fact that using a person's given name betokens familiarity. [Second half of 1900s]
See also: basis, on