Many a true word is spoken in jest proverbial saying, late 14th century, meaning that an apparent joke may often include a shrewd comment, or that what is spoken of as unlikely or improbable may in the future turn out to be true.
true-blue in early use (from the mid 17th century), applied to the Scottish Presbyterian or Whig party, the Covenanters having adopted blue as their colour in contradistinction to the royal red. Later (in the current sense), staunchly loyal to the Tory, or Conservative, Party; in the US, extremely loyal or orthodox.
true-love knot a kind of knot with interlacing bows on each side, symbolizing the bonds of love.
See also what everybody says must be true, morning dreams come true, what is new cannot be true, true as Troilus, twelve good men and true.