A candor affected is a dagger concealed.

[ἐπιτήδευσις δὲ ἁπλότητος σκάλμη ἐστίν.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 11, ch. 15 (11.15) (AD 161-180) [tr. Staniforth (1964)]
    (Source)

Depending on the source material, the weapon reference is either from a native Thracian (foreign) weapon (σκάλμη) -- thus translators who use "stiletto," etc. -- or a proverbial "crooked stick" (σκαμβή), referring to a Greek proverb "You can't make a crooked stick straight." See 1, 2, 3, 4 for more details.

(Source (Greek)). Alternate translations:

But the affectation of simplicity is nowise laudable.
[tr. Casaubon (1634), 11.14]

But on the other side, an Affectation of being Real, is an untoward pretence.
[tr. Collier (1701)]

The ostentation of simplicity is like a dagger for insidious designs.
[tr. Hutcheson/Moor (1742)]

In short, the affectation of simplicity is often a concealed dagger.
[tr. Graves (1792)]

But the affectation of simplicity is like a crooked stick.
[tr. Long (1862)]

An affectation of sincerity is a very dagger.
[tr. Collier/Zimmern (1887)]

But the affectation of simpleness is a dagger in the sleeve.
[tr. Rendall (1898)]

The ostentation of straightforwardness is the knife under the cloak.
[tr. Hutcheson/Chrystal (1902)]

A calculated simplicity is a stiletto.
[tr. Haines (Loeb) (1916)]

But the affectation of simplicity is like a razor.
[tr. Farquharson (1944)]

But the mere pretence of simplicity is like an open blade.
[tr. Hard (1997 ed.), (2011 ed.)]

But false straightforwardness is like a knife in the back.
[tr. Hays (2003)]

Calculated honesty is a stiletto.
[tr. Hammond (2006)]

A contrived simplicity is like a dagger.
[tr. Needleman/Piazza (2008)]