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.
The term is often used for lazy, irresponsible people.
On the other hand, someone who is “shifty” is too adept at change and isn’t to be trusted.