I've searched both online and in the TexBook, but can not find a command or package.
Note, I am pretty new to TeX / LaTeX, but am currently trying to figure out how to do things in Plain TeX.
Edit: Thanks for all the answers, but I realized that my question was ambiguous because I used the word "strikeout" which is commonly associated with the horizontal line. I am actually looking for the diagonal strikeout, like the one here, except for Plain TeX. How to strike through obliquely, e.g. to indicate cancellation?
Also, I am looking for a solution that works in Math mode.