The term "Choppy" describes a unit or army in Warhammer 40,000 (or any other game, for that matter, though it is most frequently used among 40K players) that excels in (or "wants to be in") close-quarters combat. Like the Imperial Guard lead by a particularly plucky commissar... or else. They tend to have some combination of high Weapon Skill, Strength, Attacks, and Initiative. Like its cousin Shooty, the term is borrowed from the Ork language, and the Ork race tends to excel at this sort of combat.
Depending on the user, "choppyness" and "shootyness" can be mutually exclusive (used to refer to specialists in melee or ranged attacks) or complementary (an assessment of a unit's quality at close-quarters or ranged combat, respectively).