The index to an object should always be a string:
var obj = {
a: 2;
}
obj.a = 3; //a is 3 now
obj['a'] = 4;//a is 4 now
You said options.elements[x].selector is input[name="username"] so I guess in this line:
this.inputs[options.el.find('form').attr('class')][options.elements[x].selector] = false;
What you are really trying to do is:
this.inputs[options.el.find('form').attr('class')]['username'] = false;
So you can do like this:
var sel = options.elements[x].selector;
console.log( 'sel is ' + sel );
this.inputs[options.el.find('form').attr('class')][sel] = false;
Make sure that sel is a string. You may want to try
var sel = options.elements[x].selector.value;
The .value bit retrieves the text from inside an input element.
options.elements[x].selector? What is the Syntax error?