aller-

Meaning

Intensifying prefix for superlatives: very; of them all

Synonyms

meest

Pronounced as (IPA)

/ˈɑ.lər/

Etymology

Inherited from Middle Dutch: aller-

In summary

From Middle Dutch aller-, alder-, the genitive plural of alle (“all”). The prefix thus literally translates to "of all", yielding adjectives such as allerergst (from ergst (“worst”)) which literally translates to "the worst of all". Words formed this way are thus effectively univerbations.
Compare English alder-, German aller-, Old Norse allra.