In a mental manner; an idea thought out in one's mind, as opposed to an idea spoken orally.
"She mentally rehearsed her speech before stepping onto the stage."
In plain English: Mentally means using your brain to think, learn, or solve problems instead of just reacting with your body.
"She studied mentally for hours but still forgot most of the information when she left home."
Usage: Use mentally when describing actions performed inside the brain rather than through speech or physical movement. Avoid confusing it with "orally," which specifically refers to communication using words that are heard by others.
The word mentally comes from combining the adjective mental with the suffix -ly to form an adverb. It entered English as a straightforward grammatical construction rather than through borrowing or meaning shift.