Mentally acute is a term used to describe someone who is quick-witted and has a SHARP intellect. This term can be broken down into four crossword solutions: AGILE (5 letters), SHARP (5 letters), APT (3 letters), and KEEN (4 letters). AGILE means to be able to move quickly and easily, SHARP means to be keenly perceptive or alert, APT means to be suitable or appropriate for a particular situation, and KEEN means to have a strong desire or enthusiasm. Together, these words describe someone who is mentally acute, or has a SHARP intellect and is able to think quickly and accurately.