Here are some possibilities:
* Effervescent: This is a possibility, but it's a bit of a stretch.
* Affectionate: This is possible too, though less likely.
* A shortened version of a name: It could be short for "Effie", "Effiemae", "Effe", or some other similar name.
* A nickname: It could be a nickname for someone named "Effie", "Effiemae", or even someone with a completely different name.
To help me determine what "Effy" stands for, please provide more information:
* What is the context in which you saw or heard the name "Effy"?
* Is it a person's name or something else?
* Is there any other information you can share about it?
With more details, I may be able to give you a more accurate answer.