Webster's 1828 version of American Dictionary of the English Language (Webster's 1828), the one most commonly used until the early 1900s, told me that messuage means
In law, a dwelling house and adjoining land, appropriated to the use of the household, including the adjacent buildings.Webster's 1828 gives the pronunciation as mess' uage but more modern dictionaries suggest the pronunciation as mes' wij. I suspect it's use has gone so far out of fashion that we will probably never hear it used during these modern days.
If you find unknown words from the 1800s while working on your family history, I highly recommend using one of the online versions of Webster's 1828 dictionary. If you don't find your word in one of them, try another. Because they are all transcriptions, I suspect that not every single word in the dictionary is included in the transcription.
The two versions I've found are M. Shaffer's transcription and Christian Soups' version. Another early Webster's, available at The ARTFL Project, combines the 1913 and 1828 versions.
Of course, if you can't find your word in one of the online versions, you can purchase a reproduction copy of Webster's 1828 (use a search engine to find available copies for sale) or you can try a modern dictionary and hope it includes and identifies archaic words.
Another unusual find in Thomas Smith's will was the word witness written in the old way in letters that look to us like witnefs. I was surprised to see this spelling because I thought that by the mid-1800s it was primarily used only in German handwriting.
More to come regarding Thomas Smith's will in the next post or two.