*er-
Proto-Indo-European root meaning "earth, ground." It forms all or part of: aardvark; aardwolf; earth; earthen; earthy.
It is the hypothetical source of/evidence for its existence is provided by: Old English eorþe "ground, soil, dirt, dry land," Old Norse jörð, Old High German erda, Gothic airþa; Middle Irish -ert "earth."

Sources:

• A Dictionary of Selected Synonyms in the Principal Indo-European Languages, University of Chicago (By: Carl Darling Buck) 1988.
• Etymological Dictionary of Latin and the other Italic Languages (By: Michiel de Vaan) 2008.
• The American Heritage Dictionary of Indo-European Roots (By: Calvert Watkins) 2000.
• Noun Declension in Indo-European (Sindhueuropaia Deklination Nomnes) By: Carlos Quiles.
• Online Etymology Dictionary (By: Douglas Harper)