In French, adjectives ending in -eux have a specific feminine form.
Learn how to make the feminine form of adjectives ending in -eux in French
Look at these adjectives:
Il est heureux à Paris, et elle est heureuse à Londres.He's happy in Paris, and she's happy in London.
C'est un livre ennuyeux. -Oui, cette histoire est ennuyeuse.This is a boring book. -Yes, this story is boring.
Adjectives ending in -eux in the masculine form become -euse in the feminine form.
Note that many of the English equivalents end in "-ous"
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Examples and resources

C'est un sujet sérieux. -Oui, et une cause sérieuse.It's a serious matter. -Yes, and a serious cause.
Il est amoureux d'elle, et elle est amoureuse de lui.He's in love with her, and she's in love with him.
C'est un livre ennuyeux. -Oui, cette histoire est ennuyeuse.This is a boring book. -Yes, this story is boring.
C'est un film merveilleux, avec une actrice merveilleuse.This is a marvelous film, with a marvelous (female) actor.
Il est heureux à Paris, et elle est heureuse à Londres.He's happy in Paris, and she's happy in London.