Someone who is acting cowardly is showing no courage. A politician who is afraid to tell the truth about his shadowy past can be described as cowardly.
Use the adjective cowardly to talk about a person whose knees knock together with fear, especially when he or she is afraid of doing the right thing. A famous example of a cowardly character is the Cowardly Lion from "The Wizard of Oz," who pretended to be ferocious but was actually lacking bravery.