keener. 1  1 : having a fine edge or point : sharp a keen knife. 2  2 : having or showing mental sharpness a keen observation. 3  3 : very sensitive (as in seeing, smelling, or hearing) keen eyesight. 4  4 : full of enthusiasm : eager … everybody was very keen to hear details of what had happened … — J. K. Rowling, Goblet of Fire.