I came across the word "arpent" in an old deed at the Ralls County, Mo., Courthouse. The reference was in this context: "800 arpents on Spencer Creek."
Wikipedia defines arpent as "a unit of length and a unit of area. It is a pre-metric French unit based on the Roman actus."
Convert-me.com offers a means to convert arpents into more commonly defined acres.
1 arpent equals 0.8448 acre
800 arpents would be 675.8 acres.