ElasticSearch, multi-match with filter?

Depending on what you need you have to put the filter in the proper position. You have two options:

Use a top-level filter and apply the filter only to the search results but not to the facets

{
    "query" : {
        "multi_match" : {
            "query" : "this is a test",
            "fields" : [ "subject^2", "message" ]
        }
    },
    "filter" : {
        "term" : { "username": "slimkicker" }
    }
} 

Use a filtered query and apply the filter to both the search results and the facets

{
    "query" : {
        "filtered" : {
            "query" : {
                "multi_match" : {
                    "query" : "this is a test",
                    "fields" : [ "subject^2", "message" ]
                }
            },
            "filter" : {
                "term" : { "username": "slimkicker" }
            }
        }
    }
}