Signalling to another via the medium of text the fact you are carrying out an action, in this case, the movement of raising the shoulders, which highlights the point you are basically saying 'I don't know' or 'whatever' but in a non-verbal manner.
Friend: Hello, how ya doin?
That kid: *shrug*
Friend: Well uh... we have English class together, right?
That Kid: *shrug*
Friend: CAN YOU PLEASE STOP SHRUGGING
That kid: *shrug*
Friend: ...You wanna come to my house?
That kid: HELL NAH
Shrugs are all the rave among teenage girls. These alleged pieces of clothing provide neither warmth nor coverage. It also fails to increase the perception of chest size. The title derives from the action spurred by a girl wearing it. You shrug because you have no idea what it is for.
When I woke up in Spanish class today, I was surrounded on eight sides by girls wearing shrugs. I could only help but shrug.