in the disguise of someone—
1. disguised or dressed as smb. else:
- There was a masquerade, in which Charles and some of his courtiers appeared in the disguise of satyrs.
2. using a specific guise as a means for smth.:
- He had appeared in the disguise of a cop for two reasons. He had been certain that the commissioner would not release any detailed information…
under the disguise of someone — = in the disguise of someone 2:
- The thief gained entry to the premises under the disguise of an inspector from the Electricity Board.