"Keen eyne" is an old English phrase that means sharp eyesight or keen vision. Nowadays, this phrase is rarely used, and there are plenty of synonyms that one can use instead. Some of the synonyms for "keen eyne" include sharp vision, clear eyesight, good eyesight, strong eyesight, excellent vision, powerful vision, accurate vision, and piercing eyesight. These synonyms can be used in multiple contexts, such as describing someone's impressive vision or discussing the effectiveness of corrective lenses. Ultimately, "keen eyne" may have fallen out of favor, but there are plenty of words that one can use to convey the same idea.