The act of urinating on another person's plot of land because there are no other trees to be found on public property.
YTMND Person 1: So I was tresp-ssing on someone's property...
YTMND Person 2: You were Trespissing?
YTMND Person 1: ...
To urinate illegally on property that doesn't belong to you.
I was walking home from the bar last night and I needed to go so bad, I had to stop and trespiss behind the elementary school.