The classic safe word is “RED”. It isn’t a word you might say in conversation or as a reaction to pain. Your safe word needs to be something you can remember in a stressful situation. Something short and simple is my advice.
My safe word is “scarlet”. i have never needed to use it. i only play with partners i know and trust. i play with partners who clearly know my limits and boundaries before we play. If i am concerned about something, i ask if we may talk, please. The code word for this is to say “yellow”. If you are to be in a situation that you cannot speak, have agreed upon hand or foot signals worked out before play begins.