confound

to confuse, mix up, or bewilder

Roots

com-: with, together, complete, or intensified
fund / fus: pour, pour out, or melt into a flow

Other words with the fund / fus root

confuse: to make unclear or hard to separate
diffuse: spread out; wordy or not concentrated
fuse: to join or melt together
fusion: a joining or blending into one
futile: unable to produce a useful result
infuse: to fill or soak with something
profuse: abundant, lavish, or poured out in large amount

Confound pours things together until clean judgment gets hard: separate causes, ideas, or signals become one muddle.

The extra variable confounded the results because no one could tell which change had helped.

Confound is stronger than ordinary confuse. The older picture is mixing by pouring together. When evidence is confounded, the separate streams have blended and the mind cannot easily sort them again.