WordWeb Online

1. Not funny; especially failing to achieve the intended humour
   "a very unamusing joke";
   - unfunny
    
2. So lacking in interest as to cause mental weariness
   - boring, deadening, dull, ho-hum [informal], irksome, slow, tedious, tiresome, wearisome, deadly [informal], draggy [informal], mind-numbing

See also: humorless [US], humourless [Brit, Cdn], lame, lameo [informal], meh [informal], samey [Brit, informal], unhumorous, uninspiring, uninteresting