I've already checked some dictionaries but can't get it clearly.
Is this expression only for bad luck?
I think I heard someone said he had to keep a stiff upper lip when he really wanted to laugh in a serious meeting.
Would you use this when you hold your laughter?
sit on one's hands [i.e. don't applaud] - show no approval
bite one's tongue - not say something that one really wants to say
stifle a yawn/giggle/sneeze - not show boredom/laughter/sneeze