Solomon Grundy is a fictional character from a popular nursery rhyme, but the phrase has also become a common idiom for something that is repetitive or routine. There are a variety of synonyms that can be used to describe this concept, such as "mundane," "routine," "monotonous," or "tedious." Other options include "repetitive," "unchanging," "dreary," or "boring." Some might prefer using more colorful phrases such as "groundhog day," "same old song and dance," or "rat race" to express this idea. Whatever the terminology used, it's important to remember that sometimes routine can be comforting, but it's also important to mix things up and keep life interesting.