bootstrap/scss/utilities/_sizing.scss

// Width and height

@each $prop, $abbrev in (width: w, height: h) {
  @each $size, $length in $sizes {
    .#{$abbrev}-#{$size} { #{$prop}: $length !important; }
  }
}

.mw-100 { max-width: 100% !important; }
.mh-100 { max-height: 100% !important; }