Dip Class
Dip Class is a piece of gamer slang that emerged into the /tg/ world as a result of Dungeons & Dragons 3rd Edition, although seeds of it had been sown in its precursor, Advanced Dungeons & Dragons. Born out of the conflux of multiclassing and Linear Warriors, Quadratic Wizards, the term "dip class" signifies a class that you "dip into" (only take a small number of levels from) in order to gain increased potency when combined with the features of your other, primary class. Generally speaking, the dip class is usually considered too weak to be worthwhile to follow extensively, but it has traits that mesh well with another, stronger class.
The original form of this concept came about in Advanced Dungeons & Dragons, when humans had the unique rule of "dual-classing". It was extremely common - so much so that AD&D-based video-games like Baldur's Gate often have guides advising players to do so - to have player start out as a fighter, for those precious low-level hit points, THAC0 and weapon slots, and then dual-class to Wizard, which would allow you to access its superior end-game abilities.
3e invented this concept, and Pathfinder arguably perfected it, what with the latter's enormous roster of classes and their wide swing of usefulness.
The concept almost died in Dungeons & Dragons 4th Edition, due to the AEDU System and how multiclassing & hybrid classing worked; it did still happen, but it wasn't quite as prevalent (or as cheesy) as it had been in the last edition.
Dungeons & Dragons 5th Edition brought it back, but at least added some depth to it; most now recommend dipping into a secondary class at least enough to get its first subclass features. Also, caster/caster and martial-primary/caster-dip dip-classing has become more prevalent; the paladin with a few levels in sorcerer or warlock is widely considered one of the best class combos in 5e.