Two words...

on·er·ous
–adjective

1. burdensome, oppressive, or troublesome; causing hardship: onerous duties.
2. having or involving obligations or responsibilities, esp. legal ones, that outweigh the advantages: an onerous agreement.

[Origin: 1350–1400; ME <>onerōsus, equiv. to oner- (s. of onus) burden + -ōsus -ous]

or·ner·y
–adjective, -ner·i·er, -ner·i·est. Dialect

1. ugly and unpleasant in disposition or temper: No one can get along with my ornery cousin.
2. stubborn: I can't do a thing with that ornery mule.
3. low or vile.
4.inferior or common; ordinary. [Origin: 1790–1800; contr. of ordinary]

Damn. They aren't related.