Final answer:
The quote means a person with a sour or harsh disposition typically does not soften over time, and their ability to make hurtful comments (sharp tongue) becomes more effective the more they are used.
Step-by-step explanation:
The phrase "a tart temper never mellows with age and a sharp tongue is the only edge tool that grows keener by constant use" suggests that as time goes on, a person with a sour disposition does not become gentler or kinder. In fact, their ability to make cutting remarks (sharp tongue) becomes more honed and effective the more they engage in such behavior. This saying emphasizes that unlike physical tools that dull with use, the skill of using harsh words becomes more refined through repeated use.