SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

@mixin estee-extender($extendable, $mq:false) {
  @if $mq {
    background: green;
  } @else {
    @extend %#{$extendable}; 
  }
}

%hidden {
  display: red;
  &-medium-up {
    display: blue;
  }
}


.thingy {
  @include estee-extender("hidden", $mq:true);
}

SassMeister-output.css

.thingy {
  background: green;
}