No Internal Message
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"No Internal Message" is commonly abbreviated "NIM". It can be used at the end of a subject line of an email or online forum posting to indicate that the entirety of the message is contained in the subject line. For example, an email subject line might read, "Will be home by 6 pm; NIM". This saves the message recipient the trouble of having to click or otherwise select the message to see if there is any additional information.
Also commonly used is "n/t", which stands for "no text".