I was told in another post that the origin of 'sobremesa' may have originated from table-cloth as 'sobremesa' can be directly translated to as over-table. I would like to learn more about the origin or development of 'sobremesa. If someone could share it history or even any corpus/famous song about 'sobremesa' it would be great!
The word sobremesa has meant several things. The primitive meaning is the least figurative: it means a cover that is put over a table; later it evolved to include ornaments that were placed on the table while not in use (e.g. reloj de sobremesa, "tabletop clock").
Another meaning was "dessert". The current Spanish word for "dessert", and for the time after the main meal where one eats dessert, is postre, cognate with Latin posterus "following" and with Spanish postrero "last". This meaning of sobremesa has disappeared (though it persists in modern Portuguese, which is closely related to Spanish).
The prefix sobre- primarily means "over, above" (Latin super), but it also means "after" and "additionally" by extension; mesa, besides "table", refers figuratively to the food that is served on the table, and to the occasion of eating a meal. So sobremesa, by associating these two figurative meanings, came to mean "the time after a meal", "the extra time spent at the table after a meal" and "the extra food consumed after the main meal".
These meanings seem to exist already in early modern Spanish. In Reloj de príncipes by Fray Antonio de Guevara (1529-1531) we have sobremesa as "time after the main meal" presented as a time where people joke around and speak without seriousness:
Presupuesto que son tales los que lo dizen y adonde lo dizen, que es sobremesa, dignos son de perdonar, pues hablan no según los libros que avían leýdo, sino según los manjares que avían comido; porque muy poco sabe de burla el que no toma lo que se dize sobremesa de burla.
(This means, in case you find it difficult, that you must forgive and forget mockery after a meal, i.e. during sobremesa, because people don't talk then "by the books they've read, but by the victuals they've eaten".)
In other texts sobremesa is used as adverb, meaning "after a meal":
E aquellos caualleros que allí çenauan, algunos se fueron luego, e otros quedaron hablando sobremesa... (Gonzalo Fernández de Oviedo, Batallas y quinquagenas, 1535-c.1552)
Y, madre, por tu vida, que sobremesa, ya que hemos comido, cuentes al señor Grajales y a mi prima el cuento de lo que te acaeció... (Feliciano de Silva, Segunda Celestina, 1534)
(In these last two texts the implication is that people stay and talk sobremesa, i.e. after eating their meal.)
In a few texts it's possible to guess that sobremesa implies extra food, but the meaning of "dessert" seems to have died out soon, leaving us only with the meaning of "easy time at the table after eating".
You can look for sobremesa in the CORDE, a big digitized corpus of Spanish-language texts.