A Simple-Singer is an untrained entertainer but despite the name not required to actually sing. There are many well known and famous Simple-Singers throughout history even though most became only noteworthy postmortem.
Use as Curse word
Of note is the dominantly derogatory use nowadays in comparison to the Age of Wishes where it was a common way to describe people taking money for entertainment
A clue as to what brought this change in perspective in the common population can be found in the rising use of the same name for prostitutes. This can be aligned with the use of the word seamstress/seamster to describe an escort.
Recently it is increasingly used to describe people who have no former bardic education or ar otherwise not following Bardic Tradition, but still earn their wages through musical Essentra-Transmissions.