Whelm originates from Old English and it means to overturn or capsize a hollow vessel (a boat, a heart); to bury by wave, flood, storm, avalanche. The etymology is from the Old English hwelfan, to ‘upheave’. This explains the modern use of “overwhelmed” and “underwhelmed”.
No voice divine the storm allay’d,
No light propitious shone;
When, snatch’d from all effectual aid,
We perish’d, each alone:
But I beneath a rougher sea,
And whelm’d in deeper gulfs than he.