Abbreviation for "Teenage DickHead." - a large subset of teenagers, almost always (99.9%) male, who enjoy acting like idiots, being obnoxious, and making fun of people for no good reason.
It is speculated that these teens have contracted a viral illness called "teenage dickheaditis," which can be caught as early as 13 years old. The infection clears by age 25, if not sooner.
Sometimes a really good ass-kicking will cure the infection spontaneously.
"Teenage DickHead.... its only Teenage DickHead.....
Their all DickHeads!" (song parody)
Jenny: "What are those guys going on the corner, drinking and insulting people for no reason?"
Mary: "What a bunch of fucking TDH's! They need to get their asses kicked!"