Word meaning: A slip of the tongue which suggests how you actually feel, often humorous

That's an example of Freudian slip (wikipedia):

error in speech, memory, or physical action that is interpreted as occurring due to the interference of an unconscious ("dynamically repressed") subdued wish, conflict, or train of thought guided by the ego and the rules of correct behavior.

It is also known, technically, I suppose, as parapraxis (dictionary.com):

a slip of the tongue or pen, forgetfulness, misplacement of objects, or other error thought to reveal unconscious wishes or attitudes.

Another word for it is Lapsus.

According to Wikipedia:

A lapsus (Latin for "lapse, slip, error") is an involuntary mistake made while writing or speaking, something long studied in philology.

The word is used in quite a few languages.

A Kinsley Gaffe where a person, especially a politician, accidentally says what they really think.

Many a truth is said in jest.

Per Wikipedia, this adage is from the Cook’s Tale by Chaucer.

This is not necessarily a slip, though it often is. In some cases, the joke was planned, not a slip, but by making the joke the speaker unintentionally gives away that he or she isn’t really joking.

But it doesn’t generally apply when a joke is obviously speaking the truth. Observational humor, satire, etc., wouldn’t usually be described with this, even though the literal meaning would apply especially well in those cases.