Without further context it is difficult to say anything about this word.
Context is important. Please provide enough for us to be able to deal effectively with your question.
Your thread title should include all or part of the word/phrase being discussed.
If you just want to know the meaning of a word, try OneLook Dictionary Search first.
Never heard/read the expression/term but as a guess, if it's the opposite of "keeper" (in terms of romance or a relationship, one of the opposite sex a person would be attracted to), the term "dumper" could be reference to a person one would not want to continue a relationship with.
Or it could be an extension of the expression 'to dump on someone', so that a dumper would be a person who, in the words of the song, 'ha[s] a tendency to dump on you'. That seems possible, but I've never met the expression.