It's "COULDN'T". I "COULDN'T" care less.
I have to listen to at least one person say 'could' every day, and that's bad enough by itself without all the incorrect uses of 'literally' and 'ironic' and the constant use of 'infer' and 'imply' to mean the same thing.
If you're going to say something, say it correctly. If not, say it in a way you know will be correct. Or better yet, don't say it at all.
Between the Internet and text messaging, I swear that when I'm 50 no one under 30 will even know how to f***ing spell.