Yeah, yeah, I know lots of people do it. And lots of people sound ignorant doing it. Yeah, there are even some rhetoric guides (like Walk to Emmaus talk guidelines) that suggest you begin this way. They're wrong. And stupid.
Now, terms need to be defined occasionally; however, two things happen when you begin with a quotation from a dictionary.
1. You forfeit your authority. You are the expert standing up there. We are giving you our attention and we expect you to act as if you deserve it. It is up to you to define your terms either by illustration (which would be interesting) or by your own authority (which conditions us to accept your authority for the rest of the talk). When you start by quoting the dictionary, you admit you aren't really in command of your material. You act like a schlub. Why should we listen to anything else you have to say?Here endeth the lesson.
2. When you begin with a dull bit of technical hoo-hah like a dictionary definition, everyone's brains turn off until the painful dullness is over. They're waiting for something interesting to happen again. But that means you've forfeited their attention, and now you've got to work twice as hard to gather it up and prove you're worthy of it. In all ways you can imagine, beginning with a dictionary definition is a weak opening.