1.
reserves; reserve army
Literally, military reserves — soldiers held back from active deployment but available if needed.
予備軍を投入した。
They sent in the reserves.
予備軍の兵士が招集された。
Reserve soldiers were called up.
両国は緊張緩和のため、予備軍の削減に合意した。
The two countries agreed to reduce their reserve forces to ease tensions.
2.
at-risk group; borderline cases; pool of potential X
Figurative and now far more common: a group of people not yet diagnosed or classified as X, but on track or at risk to become so. Standard in health and social-policy reporting.
糖尿病予備軍と言われた。
I was told I'm at risk of becoming diabetic.
若者の失業者予備軍が増えている。
The pool of young people at risk of unemployment is growing.
健康診断の結果、高血圧予備軍に分類されたので食事に気をつけている。
My checkup classified me as at risk for high blood pressure, so I'm watching my diet.
Compound of 予備 (reserve/spare) + 軍 (army/group). The figurative sense (sense 2) attaches to disease names, social categories, and economic statuses, naming a group not yet in the category but heading toward it.
COMMON COLLOCATIONS (sense 2):
- 糖尿病予備軍: pre-diabetic / at risk of diabetes
- 高血圧予備軍: at risk of hypertension
- 認知症予備軍: at risk of dementia / MCI
- メタボ予備軍: at risk of metabolic syndrome
- ニート予備軍: at risk of becoming NEET
USAGE:
In medical writing, 予備軍 often corresponds to English 'pre-' prefixes (pre-diabetic, pre-hypertension) or 'at risk of'.