jQuery.addClass not working

Use it without dot:

 $(function(){

        $('#menuNav').hover(function(){

            $('#huh').addClass('opacity');
        }, function(){
            $('#huh').removeClass('opacity');
        });
    });

$( function() {
    $('#menuNav').hover( function() {
        $('#huh').toggleClass('opacity');
    });
});

.hover() makes many events, better to use .mouseenter(). Note also that when adding class you don't have the . (dot).

$(function(){

        $('#menuNav').mouseenter(function(){

            $('#huh').addClass('opacity');
        }, function(){
            $('#huh').removeClass('opacity');
        });
    });