CAD tip # 2963:

If you are also using spell-checking in Word, you can direct AutoCAD to use the same user dictionary as in Word. With this, your user-added words will be available both in AutoCAD and Word. Set the DCTCUST variable to point to your custom dictionary. Word custom dictionary can be usually found under: \WINDOWS\MSAPPS\PROOF\CUSTOM.DIC, or C:\Documents and Settings\username\Application Data\Microsoft\Proof\CUSTOM.DIC or c:\Users\USERNAME\AppData\Roaming\Microsoft\UProof\CUSTOM.DIC.

If you wish the spell checking process always stops on a particular word (even properly spelled), add this word prefixed with tilde (~)to your custom dictionary file using a text editor.