The phrase "twiddle one's thumbs" is a commonly used idiom that refers to doing nothing or being idle. However, there are several other synonyms that can be used to convey the same meaning. For instance, the phrase "sit on one's hands" is a popular substitute that implies inaction or lack of initiative. Another synonym is "lounge around," which suggests laziness or idleness. Similarly, the phrase "waste time" can also be used to describe a person who is not making any effort to be productive. In essence, these synonyms all imply doing nothing or being unproductive, and they can be used interchangeably to convey the same message.