Petty means trivial or unimportant. If you say that a person leads a petty life, you think that what they do is 'small' and insignificant, only concerned with day-to-day matters. It's a derogatory comment, used to compare one person's life with that of somebody who has done something more important.

I think it means to discribe someone who is always petty, so they are living the petty life. Though this is not a common phrase

i dont know but i think .. meaning A small life ..