What is the origin of the idiom "on nodding terms" and when to use it?

Its origin can be traced back to the social customs of the 18th century. In those times, it was customary for people of the upper classes to greet each other with a bow or a nod instead of shaking hands. The nodding gesture was a sign of recognition and acknowledgement, but it was not necessarily a sign of familiarity or friendship. Therefore, if two people were on nodding terms, it meant that they knew each other and acknowledged each other's presence but were not necessarily close friends or associates.