ludicrous

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Laughable or hilarious because of obvious absurdity or incongruity. See Synonyms at foolish.

Century Dictionary and Cyclopedia

1. Serving for or exciting sport; laughable from singularity or grotesqueness; adapted to cause sportive laughter or ridicule; absurd.
2. Synonyms Funny, Comical, Droll, Ludicrous, Ridiculous, Laughable. Either the direct action of laughter or a corresponding sentiment is included in the signification of all these terms.
3. In this respect laughable is the generic word, but it is also one of the strongest. Funny is the weakest of the list, ranging from the meaning of ‘amusing’ or ‘odd’ down to its colloquial use in the sense of ‘strange.’ Comical still retains a faint suggestion of its origin in connection with the drama, being primarily used in connection with something done or seen, and hence something viewed by the mind: a comical predicament is just such as would be fit for exhibition in a comedy. Droll especially implies the odd or unfamiliar: as, a droll story, idea, fellow. Ludicrous is an advance in strength upon comical, as comical is an advance upon funny. Ridiculous is the only word in the list that throws contempt or even discredit upon the person concerned: it is allowable to tell a ludicrous story about one's friend, but not a story that makes him appear ridiculous. A thing may be ludicrous, etc., on account of its unreasonableness or violation of common sense; if it is ridiculous, it is certainly on that account. That is laughable which simply provokes a hearty laugh.

Wiktionary

1. adj. Idiotic or unthinkable, often to the point of being funny.

GNU Webster's 1913

1. adj. Adapted to excite laughter, without scorn or contempt; sportive.
2. adj. Ridiculously absurd.

WordNet 3.0

1. adj. incongruous;inviting ridicule
2. adj. broadly or extravagantly humorous; resembling farce