“She wasn't that engaging of a speaker.”

You went to a talk given by an author last night. You are describing the speaker, who you didn't enjoy very much. This is your feedback.

She wasn't that engaging of a speaker.

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not that (adjective) of a (something)

This is an expression that you use when you want to say that something (like a "person") doesn't have very much of a quality or characteristic (like "niceness"):

not that nice of a person

The meaning of this is similar to:

not a very nice person

But "not that ___ of a ___" is more casual.

an engaging (speaker)

An "engaging" speaker is one who interests you and makes you feel involved. He or she keeps your attention.

You can also call a book "engaging". It would be strange to call a movie "engaging" unless it was an intellectual kind of movie, like a documentary.