Appt. Is the abbreviation for "appointment".
One of the many shortened words used in text speech.
person 1: hey dud watsup
person 2: notin much I got a denist appt. @ 4.
person 1: dud it's 4:15 rite now
person 2: OH $#!% I 4got abot sprig 4ward! gtg!
Person 1: ha ha ok good luck