The term severe is the antonym, for Go purposes. Mild is a synonym. Underplay is another possible synonym - selling your position short - overplay going clearly enough too far the other way. Nurui implies lukewarm or wishy-washy play, not definite enough to be obviously wrong.
Not all apparently slack or mild plays are mistakes - some 'slack' plays are involved in Takemiya's style. And not all severe plays are correct, or best, or recommended. If they were, we'd all aspire to play like Sakata.
Velobici: Charles, perhaps I dont understand your statements. Close pincers place the most pressure on the pincered stone, but they also pressure the pincering stones to the greatest degree. Close pincers are good for attack as well as placing greater emphasis on the corner. Far pincers are good for defense, and place greater emphasis on the side. Perhaps this is most evident when a far pincer is also an extension from another stone on the same side of the board.
Imagist: It seems to me that "slack" and "mild" are not exactly synonymous. I have heard it said that Lee Changho has a mild style of play in that he allows his opponents to have what they want (but gains elsewhere). Slack on the other hand seems to indicate allowing one's opponent to have what they want without gaining elsewhere. The latter is more negative.
unkx80: May I ask, what is the source of these two diagrams describing mild versus slack?
tapir: When I play slack moves, often it is out of the intention to play honte / strong / patient moves to gain by attacking later. Fixing weaknesses that are not so bad or don't even exist or trying. Or moves intended as multi-purpose moves that fail to achieve even one of the purposes properly. Or moves intended to threaten sth. that really don't.