... as firm（steady）as a rock安如磐石，坚定不移 as dull as lead非常愚钝 as hard as iron坚如铁石，很严厉，很残酷 ...
权威例句
1.
His biography on his own website goes on to boast of the lead he has taken in sensible but dull organisations such as the European Movement and the electoral reform group Make Votes Count.