The root word of misunderstand is understand.
To explore the orgins of the word, the prefix 'mis-' should be split from 'understand'. The prefix 'mis-' comes from Old Norse meaning 'wrongly', - for example, the word 'mistake' came from the compound verb in Old Norse, 'mistake', meaning 'wrongly taken'. 'Understand' is also a compound verb in itself and came into being prior to the Anglo-Saxon invasions of Britain, and its components, 'under' and 'stand' mean just they do today, to stand under something, therefore to intimately know of that something, or to be close to it. To be close to something is to know it.
Combine all the parts together and its meaning is straightforward: To 'wrongly' "stand" "under" something, or to wrongly know it.
To explore the orgins of the word, the prefix 'mis-' should be split from 'understand'. The prefix 'mis-' comes from Old Norse meaning 'wrongly', - for example, the word 'mistake' came from the compound verb in Old Norse, 'mistake', meaning 'wrongly taken'. 'Understand' is also a compound verb in itself and came into being prior to the Anglo-Saxon invasions of Britain, and its components, 'under' and 'stand' mean just they do today, to stand under something, therefore to intimately know of that something, or to be close to it. To be close to something is to know it.
Combine all the parts together and its meaning is straightforward: To 'wrongly' "stand" "under" something, or to wrongly know it.