confound

Short definition from Wiktionary

Verb

1. 1
   To perplex or puzzle.
2. 2
   To stun or amaze.
3. 3
   To fail to see the difference; to mix up; to confuse right and wrong.
4. 4
   To make something worse.
5. 5
   To combine in a confused fashion; to mingle so as to make the parts indistinguishable.
6. 6
   To cause to be ashamed; to abash.
7. 7
   To defeat, to frustrate, to thwart.
8. 8
   To damn (a mild oath).
9. 9
   To destroy, ruin, or devastate; to bring to ruination.

Noun

1. 1
   A confounding variable.