couldn't care less
To not care at all; to have no interest or concern; to be apathetic.
I used to be so careless / As if I couldn't care less
ADJ.
better, careful, conscientious, english, fewer, greater, impatient, important
She was less careful with her money after she got the promotion.
VERB + LESS
consist, drink, make, see, speak, startle, work
I've been trying to drink less coffee since it was keeping me awake at night.
LESS + NOUN
administration, art, depth, extent, food, money, people, proportion
We spent less money on groceries this month than we did last month.
PREP.
about, in, in, than
She earned less than her colleagues despite doing the same job.