Whatever the case may be to cause the yawn, it inevitably will happen. But yawning in a meeting is rude and unless you are in a college style lecture with hundreds of other people, the yawn will probably A. Be noticed, B. Be frowned upon, and C. Be rude!
How to keep from yawning you ask? Well, if you have a solution then let me know because I don’t know how to suppress one of those jaw opening mouthily functions. I do, however, know some techniques to mask or hide the yawn.
The first technique involves clenching your teeth together and attempting to keep the rest of your face from twitching as the yawn escapes between your teeth. If you can clamp down your teeth tight without it contorting your face, this technique should work for most yawns that are not too powerful.
The second technique is similar to the first except instead of clenching your teeth you hold your lips together (you can bite your lip as if you’re thinking about the lecture/meeting/person talking).
The third technique is for those all-powerful yawns that consume your entire upper body. If that is the case you must make an excuse to move, either by shifting your position in your chair at the same time or bending down to pick up something/scratch your foot/etc. If this shields your face then let out the yawn. If this does not shield your face, then the only polite thing left to do is cover your open mouth.
If the meeting is extra boring/extra long/extra monotone/extra lacking in coffee and you continue to yawn and yawn, try to switch up the techniques as much as possible without appearing too shifty or disinterested.