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.