The phrase "ill with longing" denotes a strong and intense desire, craving, or yearning for someone or something. There are several synonyms that can aptly replace this phrase, such as pining, craving, aching, yearning, lusting, hankering, desiring, or craving. These synonyms can be used interchangeably to convey the same sentiment or emotion in different contexts. For instance, one can say, "She was pining for her lost love" or "He was yearning for a chance to prove himself." Ultimately, all these synonyms capture the intensity of the longing and the deep emotional impact that it has on the individual.