Extra work done by villeins for the *lord of the *manor e.g. at harvest and haymaking. The Latin word was *precarium. The days on which such work was done were known as 'boon days'; the lord would provide food, of a type fixed by custom. Meat, broth or cheese might be offered, and bread with ale (precaria madida, 'wet') or without (precaria sicca, 'dry').
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.