How and when did the habit of saying "thanks" instead of "thank you" start?

Question

Not that there is anything wrong with it; I'm just curious.

Answer 1

When people started being lazy with their
tongues. It's the same as our contractions
that we use in everyday language. I use 'em
in my typing 'cause it saves time. When did
it start? Probably with the big indian Chief
when he met Buffalo Bill Cody and he said
HOW!! 'Ol Bill new what he meant.

Answer 2

Laziness. Just like saying "goodbye" (God be with you), plane (instead of aeroplane) and so many others. The Brits called Americans "lip lazy" because of all our shortcuts, especially with grammar.