Source : Webster's Revised Unabridged Dictionary (1913)

Unobtrusive \Un`ob*tru"sive\, a.
   Not obtrusive; not presuming; modest. -- {Un`ob*tru"sive*ly},
   adv. -- {Un`ob*tru"sive*ness}, n.

Source : WordNet®

unobtrusively
     adv : in an unobtrusive manner; "messengers were moving
           unobtrusively over the jet-black mountain ranges,
           bearing confidential tidings from sheikhdom to
           sheikhdom" [ant: {obtrusively}]