1. To be completely screwed over, usually do to one's own stupidity.
2. To be in a real predicament and/or a lot of trouble, especially with a higher authority.
1. Stan: We're outta gas.
Jerry: Great goin' dumbass, I told you to fill up the tank when we had the chance.
Stan: I know, I know...now we're fucked sideways.
2. Dude, when your mom finds out that you maxed out her credit card you're gonna be fucked sideways.
(1)When one makes seriously intense love to another person, enough so that the one on the receiving end of the love-making can't walk straight.
(2)Drinking way too much to the point where one can't walk straight.
I just saw your mom getting fucked sideways.