on a string

Imagery of a puppeteer manipulating a marionette by its strings. Compare pull the strings.

on a string

1. Under one's control or influence.

   The magician had the audience on a string with his fascinating illusions.

   • 1984, “No More Lonely Nights”, performed by Paul McCartney:

     You've only got my heart on a string and everything a-flutter

2. (dated) In a state of great anxiety.

• ignorants