It does NOT mean indifference. It means:
1. To be submerged, turned-over, or covered up. Or, more commonly,
2. To be so over-
taken with emotion or excitement that one
feels "turned upside down" or completely engulfed.
It does NOT mean "neutral" or BETWEEN "underwhelmed" and "overwhelmed". If
anything, "whelmed" means to be overwhelmed or just slightly less excited than "overwhelmed".