English words (I'm sure you know) can be hard to spell. So he is joking about that. He is saying that if you have a word you want to use, but you can't spell it easily, then all you need to do is find an equivalent word that is easier to spell. And that is a 'synonym'...
*Sent: Her comment was so ludicrous that we finally understood that she was joking. mordant biting or caustic in though, manner, or style; sharply or bitterly harsh *Syn: acrimonious, acidulous, sardonic, scathing *Sent: The actor was upset by the mordant criticism of the gossip...
2Social and Cyber Threats Your girlfriend might be making threats to you indirectly, either through friends or online. These threats may be either emotional or physical threats that she says in a joking manner to friends. Most often, you’ll find out from concerned friends. Confront her, tell...