Jargon; most commonly found among enforcement officers, military, and those in similar circles, including criminals.

Used to describe when a tense situation, usually an operation of some kind, suffers a catastrophic breakdown and devolves into near-chaos, usually requiring violence and/or aggression to restore order.

Likewise, to say something "almost went sideways" means that such a breakdown was narrowly averted.
1) The drug bust went sideways when the dealer found the undercover cop's wire.
2) The bank robbery almost went sideways when someone set off an alarm, but the robbers got their money and got clear before the police could respond.
3) "Man, I hope this chopper patrol doesn't go sideways on us."
by Just A Dude April 3, 2012
to tell someone to hurt themselves, very badly; a curse.
When the ginormous corporation's troll fires you, you can say "go fuck yourself sideways".
by Pinkie_Flamingo November 1, 2010