What happens if you try downloading and installing the paclet manually, like so
PacletInstall[
URLDownload[
"http://pacletserver.wolfram.com/Paclets/WordData_Definitions-10.0.25.paclet",
"WordData_Definitions-10.0.25.paclet"], "IgnoreVersion" -> True]
If that doesn't work, it might be worth downloading the paclet file with a browser, extracting it manually (change the extension to .zip and unzip) and inspecting the contents.
Do you have any antivirus programs running or anything else that might interfere with the download?
By the way, the MD5 checksum of the file should be
In[2]:= FileHash["WordData_Definitions-10.0.25.paclet", "MD5", All, "HexString"]
Out[2]= "a3cd0e5aa3c45b5519aed34f88582212"
Is that the case for your download?