look up any word, like blumpkin:
Having or relating to sexual relations between multiple personalities inhabiting the same body. From "brian", referring of course to the mind, and "slash", referring to stories containing sexual encountwers between to people of the same gender.
"Is there any good Jekyll/Hyde brainslash out there?"

"Yeah! Actually, I just read some the other night!"
by ScaramoucheBlack November 20, 2009