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The etymology of the word "conserve" in the English Wiktionary Is:

From Old French conserver, from Latin conservare (“to keep, preserve”), from com- (intensive prefix) + servo (“keep watch, maintain”).

The prefix "con" means "with" or "together".

So, you can see two different meaning of "conserve": one is "to keep watch together" and the other is "to maintain together".

In the XVIth century, the French verb "conserver" meant "to sail together, staying at viewing distance of each other". This is in this sense that the ship accompanying the Jeanne d'Arc was call the "conserve". In this sense of the word "conserve", there is the French expression "de conserve", still in use today, even if rarely, meaning "going together".

In the sense of "to maintain together", you have for example this meaning for the English word "conserve":

A jam or thick syrup made from fruit.

And in French, you have "boîte de conserve" (tin can) or "faire des conserves" (to make canned food).

In English, the verb "to observe" takes from "to keep watch", while "to preserve" takes from "to maintain".

When you look at the latin it is actually obvious. So: guarding or serving together with.