The phrase "is sick" can be replaced by various synonyms depending on the context of the sentence. For instance, if you're referring to someone who's feeling unwell, you can use the words "is ill," "is ailing," "has fallen ill," "is under the weather," or "is feeling poorly." Similarly, if you're talking about something that's not functioning properly, you can use the phrases "is broken," "is faulty," "is not working," or "is malfunctioning." Additionally, if you're referring to someone who's disgusted or tired of something, you can use the words "is fed up," "is nauseated," "has had enough," or "is sickened." Therefore, by using the appropriate synonym, you can replace the phrase "is sick" with a more expressive and accurate expression.