别对她笑了牙上韭菜走光了
[bié duì tā xiào le yá shàng jiŭ cài zŏu guāng le]
This nickname implies that a person should not smile or laugh at someone too obviously, otherwise their true intention (often unrefined or inappropriate) will be exposed. 'Tooth and Chinese leek' is used humorously here, hinting that when you show your teeth (laugh), the 'unmentionable things' will come out.