To be put in one's place whilst discussing technology, by another person who's tech knowledge far surpasses your own, likely after wrongly trying to impress others with your own mediocre tech knowledge.
This can be thought of as the non-physical, tech analogy to being "bitch slapped". Though not necessary, greater effect can be realized, by starting the Tech Slap with the word, "Actually". The more in-depth, technical knowledge given, the harder the Slap.
Slappee: "Did you know that the latest satellites can read a license plate on a moving car?"
Slapper: "Actually, government satellites have been able to do that since 1984 with the NSA's May launch of Uberview. Today's satellites, with their multi-iris, 45 Megapixel, full spectrum CCD sensors could now be used to determine the sex and eye color of a second trimester fetus."
Observer to Slappee: "Dude, you just got Tech Slapped."