"Tiefling" was coined by Wolfgang Baur, when original Planescape designer David "Zeb" Cook asked for a Germanic-sounding word for humans with fiendish blood. Baur derived the name from teufel, or "Devil" in German. The direct translation of tiefling, however, would be "deepling," since tief means "deep." A closer derivation from teufel would be teufling.
In a standard game, Tieflings tend to have an unsettling air about them, and most people are uncomfortable around them, whether they are aware of the tiefling's unsavory ancestry or not.