Why do people say "truth will out"?

Solution 1:

It is idiomatic, a the similar expression “murder will out” is from Chaucer (14th c.). The sense appears to be from “out” used as a verb here meaning:

Out:

sense of "disclose to public view, reveal, make known" has been present since mid-14c.

(Etymonline)

Truth will out:

is from Shakespeare's The Merchant of Venice, 1596:

LAUNCELOT: Nay, indeed, if you had your eyes, you might fail of the knowing me: it is a wise father that knows his own child. Well, old man, I will tell you news of your son: give me your blessing: truth will come to light; murder cannot be hid long; a man's son may, but at the length truth will out.

(Phrase Finder)

The phrase "truth will out", or "truth will become public", appears as early as William Shakespeare's works, in particular, the Merchant of Venice. It may have been an entirely new concept of Shakespeare's, as he sees the need to explain its meaning as analogous to murder will out.

(Wiktionary)

From Chaucer, "Nun's Priest's Tale," c.1386:

Mordre wol out that se we day by day.

(Etymonline)