"Engaged" is a new word and idea in Much Ado.

This mutual acceptance is prefigured in Benedick's words earlier:"Enough, I am engaged." (IV.i.331) The Merriam Webster Dictionary claims that the first use of "engaged" in English is 1629, which is twenty years after Shakespeare used it here. Benedick, Josha Crowley; Beatrice, Courtney Levine.