Dubcon is a tag seen especially related to fanlore or fanart, and it is the composite of the words dubious and consent, describing a sexual intercourse that may or may not have happened with consent.
It is a middle ground between consentual sex and rape, although it is not always depicting sexual content, as presented by the common tag dubcon cuddling.
The term’s origin is hard to pinpoint, as it has been a part of the internet lingo for quite some time.
Dubious consent had appeared in Harry Potter related threads as early as 2003.
The shortened version, “dubcon” was first mentioned in 2005, alongside with noncon, which refers to non-consentual.
The term appeared on Urban Dictionary for the first time in 2006, with a couple of other definitions added since that time.
Since then, “dubcon” had spread to all kinds of fanart platforms from DeviantArt, to Ao3 to even Tumbr.