There are several synonyms for the term "hard-boiled". One of the most common is "tough", which implies a certain level of resilience and grittiness. Other synonyms might include "stern", "unemotional", or "impassive", which all suggest a lack of sentimentality or emotional attachment. "Rigid" and "unyielding" are also appropriate synonyms, as they suggest the idea of something that is inflexible or unresponsive. In some contexts, "hardened" or "jaded" might be appropriate, as they suggest a certain level of cynicism or disillusionment. Regardless of which synonym is chosen, all suggest a level of toughness and hardness.