A language whose design encourages UnmaintainableCode?, if not out-and-out IncomprehensibleProgramming?.
The CanonicalExample? of this is PerlLanguage, whose "ThereIsMoreThanOneWayToDoIt" philosophy has the unfortunate corollary "So to do maintenance, you have to learn every blasted one!"
Perl as a language has less a design than a thousand special features flying in close formation.
Ever tried writing out a COBOL program with a pencil? That's painful ;-) --
(I threw out my blank COBOL coding sheet pads only 4-5 years ago. But a pencil? Real COBOL coders use pen, and then only when there is no keypunch free!) -- JimRussell
If you have, please add yourself to HadToUseCobol!
COBOL itself strikes me as only mildly painful within its problem domain (but then, I've never HadToUseCobol, so take that with a GrainOfSalt) - now, socket programming with CobolScript, on the other hand... -- AlistairYoung
There are also deliberately painful languages, like UnLambdaLanguage, MalbolgeLanguage, InterCal, and BackWardLanguage.
Unlambda is not deliberately painful. Unlambda is 100% pure combinator graph reductor, so it just isn't readable. Writing Unlambda programs is not that hard: the Unlambda tutorial shows you how to do it. From then on, you just have to think in a tail-recursive way.
Nominations
See, obviously, LanguagePissingMatch (or possibly LanguageGriping?), PythonVsPerl, WriteOnlyLanguage