sly dig 

The act - of which to insult, make fun of, or make something along the lines of a negative comment - that is done in secret (hense sly) and as casually as possible, acting as if they are not directing it at the person in question, but actaully are. But not as secret as to the person in question does not see it/hear it, as if they do and they know that it was directed at them due to recent events, they announce it as a sly dig.

(just after Steve has been whining about not being able to get a girlfriend) Dereck: "I really hate people who complain about being single". - this being the sly dig

sly dig by rararoo February 23, 2011

sly dig 

when you say something obviously directed toward someone without actually saying their names.

Sly dig-er: "Going on "Emptees" and purchasing T shirt designs and putting them on shirts doesnt I'll repeat doesn't make you a clothing company."

sly dig by obtaininghappiness September 19, 2012

sly diggin' 

purposely talking about someone who's friends with you on facebook or follows you on twitter/instagram/tumblr.

friend one: yo, did you see that she was sly diggin' her ex on twitter
friend two: yeah, i really don't why she's still doing that. it's been a year after their break up

sly diggin' by Amelia Bottomly January 8, 2013