A curse is a word which threatens the other person to have bad luck. Nobody knows if it really works or become true. Some people believe in them even so, that they become true by magic. For example, for fairy tales, the princes are cursed to become a beast or a frog (i.e. frog prince) by witches because of their bad behaviours. But in the end a princess saves them.
There was also something called a "curse tablet" which Romans used to curse people they did not like. They would often let them drift to the river so that the curse will be carried to where that person who has the curse. But many curses are made when people are jealous and they want that person who has the curse to not do well.
The opposite of a curse is a blessing.
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