The Collaborative International Dictionary of English v.0.48:

Tacit \Tac"it\, a. [L. tacitus, p. p. of tacere to be silent, to
   pass over in silence; akin to Goth. [thorn]ahan to be silent,
   Icel. [thorn]egja, OHG. dag[=e]n: cf. F. tacite. Cf.
   Reticent.]
   Done or made in silence; implied, but not expressed; silent;
   as, tacit consent is consent by silence, or by not
   interposing an objection. -- Tac"it*ly, adv.
   [1913 Webster]

         The tacit and secret theft of abusing our brother in
         civil contracts.                         --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tacitly
    adv 1: in a tacit manner; by unexpressed agreement; "they are
           tacitly expected to work 10 hours a day"