A short hand expression for stealthily stating that something excites you so much that you achieved an erection, or "Set my Dick on Hard". This is a thinly veiled reference to a Star Trek phaser where a person could set different power levels for that weapon, with kill being the highest setting. Settings one's dick on hard would be the highest level of excitement a person could generate via their phallus.
Matt: Did you see the picture on that new 1080p Plasma TV I bought?
Steve: Fuck yeah dude, that thing set my D on H.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"