这是因为-ness后缀取的是它形容词的意思.
op·po·site
adjective
1.situated,placed,or lying face to face with something else or each other,or in corresponding positions with relation to an intervening line,space,or thing:opposite ends of a room.
2.contrary or radically different in some respect common to both,as in nature,qualities,direction,result,or significance;
3.being the other of two related or corresponding things
4.Botany.
a.situated on diametrically opposed sides of anaxis,as leaves when there are two on one node.
b.having one organ vertically above another; superimposed.
5.adverseorinimical.
名词则有限很多：
6.a person or thing that is opposite or contrary.
7.an antonym.