1 definition by marshmallowobsessed
The kid of the person you are dating that quickly becomes almost your own because you spend so much time with them outside of your dating relationship.
I am in beginning stages of dating a man with a child and I also have a child. When I go anywhere with my child it is "expected" that my instakid (the son or daughter of my boyfriend) will also join us.
by marshmallowobsessed March 26, 2013