serious

adj.

[1]	grave , solemn , earnest , unsmiling , poker-faced , straight-faced , sedate , sober , pensive , thoughtful , humourless , sombre , grim , dour , severe
[2]	grave , important , vital , dangerous , weighty , significant , momentous , crucial , consequential , life-and-death/life-or-death , urgent , pressing , no laughing matter , of consequence/moment/importance
[3]	sincere , dangerous , grave , straightforward , not joking/fooling , grievous , severe , genuine , honest , life-threatening
[4]	good , acute , critical , life-threatening , bad , dangerous , nasty , perilous , alarming , grave , severe , precarious
[5]	unplayful , sober