Shakespeare was very talented at writing lengthy concepts economically, and with rhythm. :) The saying means that if you tempt (or tease, or as Blitzy says, provoke) someone who is desperate as you may get a far stronger reaction than you expect. Romeo throws this down as a challenge to Paris in the final Act. The "tempt not" pattern (verb + negative particle) is common in Germanic languages and was commonly used in older forms of English. To us it sounds old and a bit poetic, probably thanks to Shakespeare and co. Nowadays we'd add 'do' into the sentence, ie. "Do not tempt a desperate man".

Provoke not a desperate man, isn't that hard to get =)

It means that if you aggravate an angry man he won't be responsible for his actions which could be very violent

"Tempt not a desparate man" quote by William Shakespeare(Act V, Sc. III) It means that a man or women thats desperate for what ever reason it is from a to z it does not matter if they are at their weakest point and can not take anymore, so dont push them to hard .