Here are some terms common to me but maybe not to you…

Note: This list is in a constant state of revision and I will be adding to it constantly…

Appendix N
Literally, the list of “inspirational reading” in the back of the original Dungeon Masters Guide. Authors listed (among others) include: Burroughs, de Camp, Dunsany, Howard (as in R.E.H./Conan), Leiber, Lovecraft, Merritt, Moorcock, Pratt and Vance.

The most immediate influences upon AD&D were probably de Camp & Pratt, R. E. Howard, Fritz Leiber, Jack Vance, H. P. Lovecraft, and A. Merritt; but all of the above authors, as well as many not listed, certainly helped to shape the form of the game. For this reason, and for the hours of reading enjoyment, I heartily recommend the works of these fine authors to you.
– E. Gary Gygax, 1979, AD&D Dungeon Masters Guide, p. 224

The first-edition AD&D, Dungeon Masters Guide (first published 1979)

French for “the grumblers or complainers,” Napoleonic for “the Old Guard,” a grognard is a peculiar and picky beast of the geekmosphere.  Prone to fits of nostalgia, a grognard will go out of its way to explain, on philosophical and often moral grounds, why certain out-of-print versions of games are superior to the latest off-the-shelf equivalents. They will turn their noses to games (and players) who prefer the latest edition (or even a slightly later edition) of their favourite game or franchise. For example, they may enjoy the odd retroclone RPG, but nothing’s going to compete with the thrill and sense of wonder they experienced with Moldvay/Cook D&D in their best friend’s basement back in 1981.

MM and MM2
The first-edition AD&D, Monster Manual and Monster Manual II (first published in 1977 and 1983, respectively)

Generally, a stupid or ignorant person. In geek culture a “mook” is an expendable or insignificant character (like the “red shirts” in Star Trek or lowly henchmen in D&D) or easily defeatable minions of some “evil boss,” such as goons, low-level monsters, etc. In AD&D nomenclature, a “mook” is a 0-level (or creature with less than one HD) NPC or Monster. The 1E AD&D rule that states that a fighter is entitled to a number of attacks equal to his/her level against 0-level and monsters with less than one hit die (pp. 25 PHB) is often referred to as the “mook rule.”

OSR or “Old School Renaissance” or “Old School Revival
A movement in tabletop RPG culture (that may or may not even exist or remain relevant today) where people prefer games of a certain vintage over later, more “modern” games of the same type or franchise. Such people are often referred to as grognards (see above).  The OSR also often refers to the proliferation of newly published games that closely resemble and are often compatible with said original games from the past. These new rulesets are called retroclones. Not all proponents of the OSR are grognards by generation – some are newcomers to the hobby who prefer the quick and gritty, narrative-emergent and open-ended feel of both the retroclones and the original rule sets. Some but not all grognards have a strong predilection towards classic pulp fiction, especially of the swords and sorcery variety (see Appendix N above). Whether the OSR remains relevant or not, it will always exist in such online enclaves as dragonsfoot and odd74.

The first-edition AD&D, Players Handbook (first published 1978)

Roleplaying games such as OSRIC, Swords & Wizardry, Labyrinth Lord, Castles & Crusades and Dungeon Crawl Classics which either attempt to emulate the look/feel of the TSR-era editions of Dungeons & Dragons or attempt to be “compatible” with these now-out-of-print editions (as far as legally achievable under the Open Game License) so as to support newly published third-party campaign/adventure material.  Lately, the retroclones are striking out beyond the swords and sorcery of early D&D and into other genres (e.g. Apes Victorious, as well as Starships & Spacemen by Gobliniod Games).