IPA: /ʌnˈdjuːli/
KK: /ʌnˈduːli/
In a way that is more than is necessary, reasonable, or appropriate; excessively.
He was unduly harsh in his criticism of the project.
Unduly is formed from "un-" (meaning not) and "due" (from Latin "debere", meaning to owe or be obligated). The word means not owed or not appropriate, indicating something that is excessive or unwarranted.
Think of 'not' ('un-') being applied to something that is 'owed' or 'appropriate' ('due'). This helps you remember that 'unduly' refers to something that is not appropriate or excessive.