It’s a sign of considerable shrewdness to be able to make others think one is not exceptionally shrewd.

[C’est avoir fait un grand pas dans la finesse, que de faire penser de soi que l’on n’est que médiocrement fin.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 8 “Of the Court [De la Cour],” § 85 (8.85) (1688) [tr. Stewart (1970)]

(Source (French)). Alternate translations:

He is far gone in politicks, who begins to find he is but indifferently politick.
[Bullord ed. (1696)]

He is far gone in Cunning, who makes other People believe he is but indifferently Cunning.
[Curll ed. (1713)]

He is thorough-paced in Cunning, who makes others believe that he is no Conjurer.
[Browne ed. (1752)]

A man must be very shrewd to make other people believe that he is not so sharp after all.
[tr. Van Laun (1885)]

A man has made great progress in cunning when he does not seem too clever to others.
[Common Translation, e.g.]