There are several synonyms for the phrase "grounds of indisposition." Some commonly used ones include reasons for feeling unwell, justifications for ill health, the basis for physical discomfort, causes for malaise, and factors contributing to a lack of well-being. Other options could include motives for feeling under the weather, explanations for bodily distress, rationales for being unwell, and sources of discomfort. Ultimately, any phrase that conveys the concept of an underlying reason or cause for feeling ill could be used as a synonym for grounds of indisposition.