From Wiktionary, the free dictionary
сми́рный (smírnyj) + -о (-o)
сми́рно • (smírno) (comparative (по)смирне́е or (по)смирне́й)
- quietly (in a quiet manner)
сми́рно • (smírno)
- short neuter singular of сми́рный (smírnyj)
сми́рно! • (smírno!)
- (military) attention! Used as a command to bring soldiers to the attention position.