To mix up is to confuse one thing with another. Mix up the twins, mix up the dates, I always mix up those two.
It is an honest mistake rather than carelessness, which is why it is easily forgiven. Written with a hyphen the same words make a noun meaning the confusion itself.
1phrasemix upTo confuse one thing with another.
“People mix up the two brothers constantly.”
mix up is a phrase.
Practised in Phrasal Verbs