This is a more-or-less archaic word in the sense that it is not often used in modern times, however in the late eighteenth/early nineteenth century it was a euphemism for fool or idiot (OED).

There is no evidence for a song with this title existing. During the Georgian period wedding hymns were often sung at parties that were not weddings, just for fun. However, the lyrics for this particular hymn does not exist. Austenites speculate that the author invented this title as an excuse for John Thorpe to suggest marriage to Catherine (ODP).

The Argyle Buildings are located on Argyle Street in Bath. It is a famous street located in the center of the city that was - and still is - popular with tourists for hotels, pubs, spa resorts, and venues for social gatherings (Alexander Whitelaw, Conversations Lexicon, 464).

While in Austen's time the word "gay" could be defined as lighthearted, fun, or happy, in modern times it is more traditionally used as a synonym for "homosexual." Also, while in Austen's time it was a positive word, nowadays it is even sometimes used as a slang for something negative or disagreeable, or a slur (OED).

Greatcoats were a type of caped trench coat/ over coat that first came into use by the military in the 17th century, then evolved into casual wear for the upper class. They were made of thick wool and thus expensive for average people to own. However, by the time of the Industrial Revolution and Jane Austen's death, they were becoming more and more available to the middle and lower classes as well (ODP).