To laugh in the sleeve

Related to To laugh in the sleeve: laugh up one's sleeve
to laugh privately or unperceived, especially while apparently preserving a grave or serious demeanor toward the person or persons laughed at; that is, perhaps, originally, by hiding the face in the wide sleeves of former times.

See also: Sleeve

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.