Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

@function map-combine($source, $map) {
  @each $key, $value in $map {
    $new: ($key: $value);

    @if map-has-key($source, $key) {
      $current: map-get($source, $key);
      $new: ($key: map-merge($current, $value));
    }
    
    $source: map-merge($source, $new);
  }
  
  @return $source;
}

$map: (
  'pica' : (
    'group-a': (
      'font-size': 16px,
      'line-height': 18px
    ),
    'group-b': (
      'font-size': 18px,
      'line-height': 20px
    )
  ),
  'pica-bold': (
    'group-a': (
      'font-weight': bold,
      'letter-spacing': -1px
    ),
    'group-c': (
      'letter-spacing': 0
    )
  )
);

.pica {
  content: inspect(map-get($map, 'pica'));
}

.pica-bold {
  content: inspect(map-combine(map-get($map, 'pica'), map-get($map, 'pica-bold')));
}

SassMeister-output.css

.pica {
  content: ("group-a": ("font-size": 16px, "line-height": 18px), "group-b": ("font-size": 18px, "line-height": 20px));
}

.pica-bold {
  content: ("group-a": ("font-size": 16px, "line-height": 18px, "font-weight": bold, "letter-spacing": -1px), "group-b": ("font-size": 18px, "line-height": 20px), "group-c": ("letter-spacing": 0));
}