Freudian Slip
Etymology
From Sigmund Freud, whose "The Psychopathology of Everyday Life" (1901) argued that such errors, technically called parapraxes (German "Fehlleistungen"), are not random but expressions of repressed desires. Popular usage adopted "Freudian slip" for any embarrassing, seemingly revealing misstatement. Whether such slips truly disclose the unconscious remains contested, but the phrase endures.
noun phrase · /FROY-dee-uhn SLIP/
A verbal mistake thought to reveal an unconscious thought or wish.
What it names
For the telling error — a slip of the tongue read as a window onto what someone really thinks.
Example
She called her boss "Dad," then went red — a classic Freudian slip.
Related idea
A Freudian slip is an error read as revealing hidden truth; a malapropism is the comic misuse of a similar-sounding word with no deeper meaning — the telling mistake versus the merely funny one.