Trespass  —v. 1 (usu. Foll. By on, upon) make an unlawful or unauthorized intrusion (esp. On land or property). 2 (foll. By on) make unjustifiable claims on; encroach on (trespass on your hospitality). —n. 1 law act of trespassing. 2 archaic sin, offence.  trespasser n. [medieval latin: related to *trans-, *pass1]