The most useful of all social graces is the ability to yawn with your mouth closed.

Lawrence J. Peter (1919-1990) American educator, management theorist

Peter’s Quotations: Ideas for Our Time (1977)
    (Source)

Variant: As quoted in The Wall Street Journal (9 Aug 1984): 'At board meetings, "the one unmatched asset is the ability to yawn with your mouth closed," says Robert Mueller in a new book, 'Behind the Boardroom Door.'"