D’après le Dictionnaire de l’Académie Française ce mot remonte au XIIè siècle, et viendrait du latin. Il n’est fait mention d’aucun autre sens que l’actuel.

Quant à son usage écrit : si on le compare à des mots d’usage plus courant comme neige par exemple, il semble avoir connu une heure de gloire aux XVIè et XVIIè siècles pour décliner et ne redevenir significatif qu’à la moitié du XXè siècle. Mais sans être jamais tombé dans l’oubli cependant.

Merda is a Latin word, often used, even by Horace. It was not coarse at all, they didn’t use any euphemism or circumlocution. The word and its derivatives has been used by Rabelais (Garguantua & al.) with too much delectation – perhaps it started to be rude at that time ? But even Voltaire (Oreilles & al.) used it.

Nowadays, it can be an expression of surprise and joy, just familiar : “Tu te maries !? Merde alors !”

Beware about “antan”, meaning “from last year”, and not “a long time ago” like “jadis”.
“Mais où sont les neiges d’antan ?” (the famous poem by Villon was written in Spring) means : “they were very new, and they have already vanished”

The history of merde and its uses is not easy to establish since it’s long been considered as belonging to coarse speech and therefore almost taboo. So it’s always stayed somewhat under the radar of lexicographers. However the Dictionnaire de l’Académie française has been mentioning it as a headword since its first edition but with warnings such as Les honnêtes gens évitent avec soin d’ employer ce mot dans la conversation (4th edition, 1762), Les gens bien élevés évitent avec soin d’ employer ce mot dans la conversation (5th edition, 1798), or On évite d’ employer ce mot dans la conversation (6th edition, 1835). The articles are therefore not very developed.

The meaning of merda, excrementa seems to have remained stable throughout the years.

To get an idea of its uses we have to turn to other sources than the official French dictionaries. Here’s the article we find in Randle Cotgrave’s Dictionarie of the French and English Tongues published in 1611.

Merde. f. Man’s dung, turd, excrements, ordure.

• Merde de fer. The dross of iron.
• Merde oye. A goose-turd green.
• Robbe d’argent brodée de merde. An excellent text ill-expounded, or commented on; (for which cause Rabelais calls so Justinian’s Institutes).
• A cul de foirard toujours abonde merde. Prov. There wants no turd at shitten fellows’ tails.
• Le porc a tout bon en soy fors que la merde. Prov.

The last proverb is widely known and used in today’s French in a different version, namely Dans le cochon tout est bon. It’s followed in Cotgrave’s dictionary by this extraordinary remark : Yet is the dung of a hog an excellent remedy for blood-spitting; but it must first be eaten, fried with sweet butter, and some of the bloody spittle.