This word originates from chinese dialect and is used mostly in singlish. It means "lucky" as in an exclamation after a close shave. To pronounce "heng", first say hair, then add "-ing" to make it "hairing" then take out "-ri-" so it sounds like "hairng" as in hair.
Note: the example below is in singlish. Especially if you live in a predominantly english environment, like England, do not try the example in front of the teacher. Firstly, she/he will remember about the HW, and secondly, his/her head may explode from the Singlish. Of course, this may be a good thing.
Lucky the teacher forgot about the homework we supposed to submit today cos I never do! Heng ah!