(Ex cal [ce] liber [atus]). Liberated from the stone. The sword which Arthur drew out of the stone, whereby he proved himself to be the king. (See Sword.)
“No sword on earth, were it the Excalibur of King Arthur, can cut that which opposes no steady resistance to the blow.” —Sir Walter Scott.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894