Chop or Stamp

At the Malaysian Immigration checkpoint at Johor Bahru, my children's uncle asked for our passports, to be chopped by the immigration officer.

Julian and Rebecca asked "chopped? Why chop the passports?" Their uncle did not get it, and repeated, "yes your passports must be chopped by the immigration." Now the children became seriously worried, and they asked again, "why do they need to chop my passport?" After all, having your passports chopped up is not a joke you can live with.

Twenty years ago when I worked in Australia, I asked the receptionist to give me the 'company chop' and she gave me a blank look. I repeated the request and I still could not get what I needed.

Of course, the correct word is stamp, not chop.

How and why did we say 'chop'?