When we talk about being on thin ice, we are usually referring to a precarious situation which requires caution and care. If you're looking for synonyms to use instead of the phrase "thin ice," there are several options to consider. For instance, you may opt for "dangerous ground," "precarious footing," or "slippery slope." You could also say that someone is "walking a tightrope," "treading carefully," or "navigating a minefield." Each of these phrases conjures up a sense of danger and risk, highlighting the need for caution and vigilance in challenging or potentially hazardous circumstances.