SIY is the counterpoint to DIY - Screw It Yourself. It can refer to one of two things:
1. A frustratingly difficult Do It Yourself exercise attempted by an unqualified 'handyman'.
2. A retort to a friend setting you up with a troll.
Combinations are possible.
1. Fuck this SIY Ikea shit!
2. SIY, I'm not touching that.
Combination: SIY, I bet she goes for premature ejaculators.