1
general::
Phrase(s): *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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms