PIE base ‘To move, set in motion’.
Other derivatives include arise, orient, and earnest.
The American Heritage Dictionary of Indo-European Roots suggests that this PIE base is the root of are, the plural present indicative of to be, whose origin beyond Proto-German *ar- is considered unknown by some, but also possibly from PIE base es-, root of is and am.
[Pokorny *er-3 : or- : r-, pp. 326–332 and *ergh-, p. 339, ‘to shake, tremble’.]