*er- (2) Look up *er- (2) at Dictionary.com
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."