noun [ usually sing. ]
1.
a play, concert, etc. for which all the tickets have been sold :
Next week's final looks like being a sell-out.
a sell-out tour
2.
a situation in which sb is not loyal to a person or group who trusted them, by not doing sth that they promised to do, or by doing sth that they promised not to do :
The workers see the deal as a union sell-out to management.