After someone has confided what they plan to get a common recipient as a gift, gift-jacking is stealing that person's gift idea and buying it for the recipient yourself first. Result is that they have to think of something else to give. Often followed later with the gift-jacker mentioning what they got the recipient and pretending it was their idea in the first place.
Noun form is Gift-jacker: being the person who does the idea theft.
I was going to get Jane an apron for Christmas, but I got gift-jacked by my Mom.