Why is a lazy, irresponsible person called “shiftless”?

The word shift means to change or rearrange, which is why we call
those who work during differing blocks of time “shift workers.” This use
of the word shift also applies to an individual’s ability to change or
adapt. Therefore, if you’re “shiftless” you lack the initiative or resources
to change with the circumstances. On the other hand, someone who is
“shifty” is too adept at change and isn’t to be trusted.