From The Collaborative International Dictionary of English v.0.48 [gcide]:
Tasteless \Taste"less\, a.
1. Having no taste; insipid; flat; as, tasteless fruit.
[1913 Webster]
2. Destitute of the sense of taste; or of good taste; as, a
tasteless age. --Orrery.
[1913 Webster]
3. Not in accordance with good taste; as, a tasteless
arrangement of drapery; a tasteless remark.
[1913 Webster] -- Taste"less*ly, adv. --
Taste"less*ness, n.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
tasteless
adj 1: lacking flavor [ant: tasty]
2: lacking aesthetic or social taste [ant: tasteful]