Having looked at, glanced at, surveyed, or peered at someone else's dildo, thus making the person or thing who has seen it alienated or unwanted by others.
Bud: Wow, I can't believe people actually inhabit this place, man!
"Seen it" is a shortened version of "Seen Your Arse". A person who has "seen it" has become suddenly irate or upset, often disproportionately, as a result of bad news or misfortune.
Someone's stolen Simon's pint and he's seen it, royally!