Expatiate, v. t.
To expand; to spread; to extend; to diffuse; to broaden. Afford art an ample field in which to expatiate itself. Dryden.
1 other Webster entry under this spelling
Expatiate, v. i. [imp. & p. p. Expatiated;p. pr. & vb. n. Expariating.]
1. To range at large, or without restraint. Bids his free soul expatiate in the skies. Pope.
2. To enlarge in discourse or writing; to be copious in argument or discussion; to descant. He expatiated on the inconveniences of trade. Addison.
Shades of meaning
Fernald has no entry of its own for expatiate; he lists it under amplify.
Where he writes about it:
So, in the early days of our literature amplify was used in the favorable sense; but at present this word and most kindred words are coming to share the derogatory meaning that has long attached to expatiate. — from amplify
We may develop a thought, expand an illustration, extend a discussion, expatiate on a hobby, dilate on something joyous or sad, enlarge a volume, unfold a scheme, widen the range of treatment. — from amplify