"This is the first truth that ___ thine own tongue was guilty of": "All's Well That Ends Well"

EER

Think of an old word for someone who might hesitate or waver in speech.

The clue quotes a line from Shakespeare's "All's Well That Ends Well," where the missing word is "eer," an old-fashioned term meaning "before." The answer "EER" fits because it completes the phrase "This is the first truth that e'er thine own tongue was guilty of," making it a direct fill-in-the-blank solution. The wordplay is straightforward, relying on familiarity with archaic language rather than complex tricks.