An abbreviation for 'obvious,' popular with people of limited intelligence, generally of the 13-year-old girl variety. Often used to refer to things that are only obvious by virtue of the mindless monotony of the user's life.
sorority girl - "im always willing to try something new"
guy - "in that case, stop by my house with a box of condoms"
sorority girl - "ok"
guy - "and bring a bottle of lube just in case"
sorority girl - "obvi"