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Sexual activity between members of a step-family. Nowadays, it most often refers to a sexual relationship between step-siblings. However, it can also refer to a relationship between a step-parent and a step-child.
-The two of you aren't allowed to make out with each other! That's incest!

-Correction, STEPcest. Just because our parents are married doesn't mean we're related to each other.

-Yes you are!

-Not by blood! We're step-siblings. Emphasis on step.
by EveKnight75 August 26, 2006
when step-siblings have sex
Yeah, somewhere in the next room Dick and Barbara are huddled together, worried that they could get a divorce JUST LIKE Bobby Fitster's parents and worried that it will make it really hard for them to have hot stepcest sex.
by drmutant August 07, 2006