overwhelm

Overwhelm comes from Middle English overwhelmen, formed from over- and whelmen. Whelmen traces back through Old English hwealf, itself from Proto-Germanic roots related to *hwalbaz, while over- descends from Old English ofer-, ultimately from Proto-Indo-European *upér.

The word was already a combination in Middle English, joining the 'over' prefix to whelmen, so its two-part structure has stayed intact into modern English.

Overwhelm shares its 'over-' prefix with countless other English words built the same way, though the facts here don't name a specific sibling word to compare.