Lunchbag refers to loser or a person with no friends, doesn't "fit in" and does nothing right. These were people who brought lunch to school or office in a bag, then went off to sit and eat it alone because no-one liked them.

Lunchbag（午餐盒人）指生活中的失败者或者那些没有朋友、无法融入所在的环境而且经常做错事的人。这些人通常都自己带着午餐盒上学或上班，午餐时独自一人坐在一边吃饭，因为没有人喜欢他们。

For example:

Stop being a lunchbag! Come and join us for happy hour after work!

不要自己吃饭了，下班后跟我们一起去酒吧喝一杯吧。