Though she is very feminine, " a slender, small-breasted girl with an erect carriage," Nick also describes Jordan Baker as being "like a cadet": "an erect carriage which she accentuated by throwing her body backward at the shoulders like a young cadet." This description is not just limited to her posture. Like a cadet on watch, she keeps an alert but somewhat removed eye on everything. Also, she is possessed of a cadet's wariness; Jordan doesn't often let herself slip.
Earlier in the scene, when Nick says something that temporarily breaks her veneer, Jordan is quick to recover her exterior pose. Her overly enthusiastic outburst "surprised her" and she "yawned with a series of rapid, deft movements" and "stood up in the room." Jordan, like a cadet, has momentarily stepped out of line (even while she appears to be holding it up) and quickly rejoins the formation.
The formation, in Jordan's case, is to appear to love the ease of the non-demanding "good life." However, it is clear that she is bored. Unfortunately for other characters, Jordan will relieve her boredom at their expense.