unlikely

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

• adjective Not likely; improbable.
• adjective Not promising; likely to fail.

from The Century Dictionary.

• Such as cannot be reasonably expected; improbable: as, an unlikely event.
• Not holding out a prospect of success or of a desired result; likely to fail; unpromising.
• Not calculated to inspire liking or affection; not likable or lovable.
• In an unlikely manner; with no or little likelihood; improbably.

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

• adverb In an unlikely manner.
• adjective Not likely; improbable; not to be reasonably expected
• adjective Not holding out a prospect of success; likely to fail; unpromising.
• adjective obsolete Not such as to inspire liking; unattractive; disagreeable.

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

• adjective Not likely; improbable; not to be reasonably expected; as, an unlikely event; the thing you mention is very unlikely. Not holding out a prospect of success; likely to fail; unpromising; as, unlikely means.
• adverb In an improbable manner.
• noun Something or somebody considered unlikely.

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

• adjective not likely to be true or to occur or to have occurred
• adjective having a probability too low to inspire belief
• adjective has little chance of being the case or coming about