Initialism: sudden arrhythmic death syndrome; sudden adult death syndrome; swine acute diarrhea syndrome.

缩略词：心律失常性猝死综合征；成人猝死综合征；猪急性腹泻综合征。

Plural of SAD (initialism): e.g., seasonal affective disorder; standard American diet; social anxiety disorder; single-wavelength anomalous dispersion.

SAD 的复数（缩略词）：如季节性情感障碍；标准美国饮食；社交焦虑障碍；单波长异常色散。

Colloquial plural noun: sadness; a bout of the blues.

口语复数名词：忧伤；一阵低落情绪。

Third-person singular simple present of the verb sad (archaic): makes someone sad.

动词 sad 的一般现在时第三人称单数（古）：使某人伤心。