When something or someone has "seen some shit", it vaguely means that something or someone might have gone through trouble in the past, in which said experience could result in wisdom.
The ultimate degree to which an individual is pummelled/ kicked-in/ beaten-up... The last word in a complete and utter kicking... Of UK origin but, believe me, there is no escape...
Goodness! You should've seen those old/bald geezers going mental... After reading that embarrassing and patronizing claptrap penned by Ethan P., the creaky fellas went apeshit and kicked seven shades of shit out of him. Everyone applauded, including the pacifists. That'll make the ageist cunt think twice before spouting such drivel again.