Eavesdropping is secretly listening to the private conversation of others without their consent.
In order to understand about the etymology of the given word you have to understand about the Back formation and lexeme.
A lexeme is a unit of lexical meaning that exists regardless of the number of inflectional endings it may have or the number of words it may contain. It is a basic unit of meaning, and the headwords of a dictionary are all lexemes. For example, in the English language, run, runs, ran and running are forms of the same lexeme, conventionally written as run.
In etymology, back-formation is the process of creating a new lexeme, usually by removing actual or supposed affixes. Back-formation is different from clipping – back-formation may change the part of speech or the word's meaning, whereas clipping creates shortened words from longer words, but does not change the part of speech or the meaning of the word.
1. Babysit from babysitter.
2. Bicep from biceps.
3. Biograph from biography.
4. Burger from hamburger.
From the above examples it just means that the word ‘burger’ has got originated from the word ‘hamburger’. So this was the example of back formation.
Etymology of the word ‘Eavesdropping’.
The verb eavesdrop was originally a back-formation of the noun eavesdropper ("a person who eavesdrops") which was formed from the unrelated noun eavesdrop ("the dripping of water from the eaves of a house; the ground on which such water falls"). An eavesdropper was one who stood at eavesdrop (where the water fell, i.e., near the house) so as to overhear what was said inside.
And I found it worth sharing.