Originally starting as the acronym ISHYGDDT (I sure hope you guys don't do this) on 4chan, the acronym was later extended to 'Shiggy Diggy Doo'. When the term first came into use, many newfags thought it was also an acronym, so it was often said as 'I SHIGGY DIGGY', which was implied as an acronym for 'I sure hope I go get young dogs in good gay yelps', which was a reference to the massive amount of furry pornography that flooded the site during that time. Even today, the phrase is in use on many mainstream internet sites such as 4chan, reddit, tumblr, digg, funnyjunk, and facepunch.
Guy 1: Hey dude, I just bought myself a pack of ciggs. Want one?
>Not smoking weed instead of pleb-tier cigarettes
>Shiggy diggy doo