n̥dʰer-
“under, below”below/under
PIE preposition meaning under or below. Source of Latin infra, English "under," and words for lower position.
Discussion
*n̥dʰer- is a Proto-Indo-European preposition meaning "under" or "below," one of the core spatial terms in the PIE positional system.
The root gives Old English under (modern English under) and German unter, Dutch onder—all preserving the spatial sense directly. The syllabic nasal *n̥ gives Germanic *un-. Latin infra "below" and inferus "lower" (whence English inferior, infernal, infrastructure) derive from a related form with different suffixation.
Sanskrit ádhara- "lower" and adháḥ "below" reflect the Indo-Iranian branch. The relationship between the initial *n̥dʰ- and the Sanskrit adh- is regular, with the nasal absorbed.
The root is opposed in the PIE spatial system to *upo "up from below" and *ud- "upward," forming a vertical axis. The metaphorical extension from spatial "below" to hierarchical inferiority (inferior, underling) is shared across multiple branches.
Modern descendants include English under, underneath, and through Latin: inferior, infernal, infra-, and infrastructure.
Notes
Source of Latin "infra", English "under". Vertical inferiority marker.