update-woocommerce-coupon-label.php

<?php // Don't copy me unless you need to!

/**
 * Changes the coupon label output from Coupon: {code} to Coupon: {description}
 *
 * @param string $label the cart / checkout label
 * @param \WC_Coupon $coupon coupon object
 * @return string updated label
 */
function swwp_change_coupon_preview( $label, $coupon ) {

	// WC 3.0+ compatibility
	if ( is_callable( array( $coupon, 'get_description' ) ) ) {
		$description = $coupon->get_description();
	} else {
		$coupon_post = get_post( $coupon->id );
		$description = ! empty( $coupon_post->post_excerpt ) ? $coupon_post->post_excerpt : null;
	}

	return $description ? sprintf( esc_html__( 'Coupon: %s', 'woocommerce' ), $description ) : esc_html__( 'Coupon', 'woocommerce' );
}
add_filter( 'woocommerce_cart_totals_coupon_label', 'swwp_change_coupon_preview', 10, 2 );