IPA: /ˈlɛŋθɪli/
KK: /ˈlɛŋkθɪli/
In a way that takes a long time or is very detailed, often more than necessary.
The speaker talked lengthily about the history of the project.
Lengthily → It is formed from "length" (from Old English "length", meaning the measurement of something from end to end) and the suffix "-ily" (meaning in a manner). The word "lengthily" means in a manner that is long or extended in duration.
Think of 'length' as measuring something from end to end, and '-ily' means in a certain way. So, when something is described as lengthily, it means it is done in a long or extended manner.