A short string, as short a single sentence, appended to a message.

Rare these days, this was an old tradition that became a common feature of email clients, particularly from the pre-internet (BBS) days. Blue Wave had a feature to collect them.

It was for wisdom and wise racks.

There was a generally accepted practice to have taglines fit within 70 characters (Some say that it’s 74), influenced by screen width for text terminals, but that ended up getting regularly ignored.