doubtful

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Subject to or causing doubt.
• adjective Experiencing or showing doubt.
• adjective Of uncertain outcome; undecided.
• adjective Raising doubts as to legitimacy, honesty, or respectability; suspicious.

from The Century Dictionary.

• Full of doubt; having doubt; not settled in opinion.
• Causing doubt; dubious; ambiguous; uncertain; not distinct in character, meaning, or appearance; vague: as, a doubtful expression; a doubtful hue.
• Admitting of or subject to doubt; not obvious, clear, or certain; questionable.
• Of uncertain issue; precarious; shifting.
• Of questionable or suspected character.
• Fearful; apprehensive; suspicious.
• Indicating doubt; disturbed by doubt.
• In prosody, variable in quantity; capable of being pronounced or measured either as a long or as a short; common; dichronous.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Not settled in opinion; undetermined; wavering; hesitating in belief; also used, metaphorically, of the body when its action is affected by such a state of mind.
• adjective Admitting of doubt; not obvious, clear, or certain; questionable; not decided; not easy to be defined, classed, or named.
• adjective Characterized by ambiguity; dubious
• adjective Of uncertain issue or event.
• adjective obsolete Fearful; apprehensive; suspicious.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Subject to, or causing doubt
• adjective Experiencing or showing doubt, sceptical
• adjective Undecided or of uncertain outcome
• adjective obsolete Fearsome, dreadful.
• adjective Improbable or unlikely
• adjective Suspicious, or of dubious character
• adjective Unclear or unreliable

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective open to doubt or suspicion
• adjective fraught with uncertainty or doubt
• adjective unsettled in mind or opinion