When you have been sufficienly and generously satisfied mentally. It is the zenith of mental pleasure, espcially amongst intellectuals or it can be just some funny comment or remark said or written so well that it can can make the recipients of the remark or comment say "Well that made my day"
"Man that book by Jose was so eloquent and so fantastic, it was quite stimulating and fantastic...I am mentally cummed"
"Our math teacher mentally cummed himself when he found a type in your SAT workbook"
"Network TV mentally cums on America everyday"