Hangul syllable jjam

It means spare time or experience built over time.

The surrounding expression does the sorting. 짬을 내다 means to make time; 짬이 나다, to find a free moment; and 짬이 없다, to have no spare time. 짬짬이 means using spare moments as they come. In military-derived slang, 짬 can mean time served, seniority, or know-how gained through experience.

A Korean word-history account traces 짬밥 from 잔반, meaning leftover food, through 잔밥 and 짠밥. It places 짬밥 in fiction and newspaper writing in the 1980s. The story moves from leftover food to barracks meals, then from those meals to time served and experience.

Is 짬 military slang?

It can be, but 짬 alone is not automatically military.

짬밥 and 짬찌 signal military-derived usage; 짬찌 is a put-down for a recruit or low-ranking soldier. By contrast, 짬을 내다 and 짬짬이 use the everyday spare-time sense.

The service sense is casual and insider-facing. In a formal message about professional experience, the ordinary Korean word 경력 is safer; 경험 means experience more broadly. Using 짬 can make seniority between speaker and listener part of the point.