confound

To perplex or puzzle.

And the brother of Jared being a large and mighty man, and a man highly favored of the Lord, Jared, his brother, said unto him: Cry unto the Lord, that he will not confound us that we may not understand our words.

The fightback when it came was in the [Roger] Federer fashion: unfussy, filled with classy strokes from the back with perfectly timed interventions at the net that confounded his opponent. The third set passed in a bit of a blur, the fourth, which led to the second tie-break, was the most dramatic of the match.

To stun or amaze.

To fail to see the difference; to mix up; to confuse right and wrong.

1651 (Latin edition 1642), Thomas Hobbes, De Cive (Latin title) Philosophicall Rudiments Concerning Government and Society (English), Hey who lesse seriously consider the force of words, doe sometimes confound Law with Counsell, sometimes with Covenant, sometimes with Right. They confound Law with Counsell, who think, that it is the duty of Monarchs not onely to give ear to their Counsellours, but also to obey them, as though it were in vaine to take Counsell, unlesse it were also followed.

To make something worse.

Don't confound the situation by yelling.

While she had obeyed him, smiling sweetly all the time, she had nursed a growing resentment of what she called his "Latin American macho attitude." To confound the problem, his mother, who lived with them on and off, was described by the wife as being as domineering as her son.

To combine in a confused fashion; to mingle so as to make the parts indistinguishable.

There the freſh and ſalt water would meete and be confounded together, […]

Medication and lifestyle factors significantly confound the association with obesity, for which there are few prospective studies and weak evidence for a directly causal relationship, while the association with traumatic brain injury is potentially confounded by ‘accident proneness’ or physical abuse.

A confounding variable.

The participants certainly differ in how their practice is distributed (1, 2, or 3 days), but they also differ in how much total practice they get (3, 6, or 9 hours). This is a perfect example of a confound—it is impossible to tell if the results are due to one factor (distribution of practice) or the other (total practice hours); the two factors covary perfectly.