onder-

See onder.

onder-

1. prepended to verbs; denotes that the action happens underneath, or downwards on the object; sub-, under-, lower

   Antonym: over-

   ‎onder- + ‎lijnen → ‎onderlijnen

2. prepended to verbs, nouns and adjectives; denotes that the action or state takes place insufficiently

   Antonym: over-

   ‎onder- + ‎presteren → ‎onderpresteren

The second meaning is not shared by compounds with the adverb onder.