adjective

lacking humour; not able to appreciate or express humour.

they are such a humourless bunch

serious,serious-minded,solemn,earnest,sober,sombre,grave,stern,grim,dour,morose,unsmiling,stony-faced,gloomy,glum,depressed,sad,melancholy,dismal,doleful,mournful,dejected,despondent,joyless,cheerless,lugubrious,in low spirits,with a long face,boring,tedious,dull,dry,heavy-going