Contronyms are what you're referring to. Indeed, flammable/inflammable, also sanction/sanction (permit/punish) and other examples such as fast/fast (going quick

Contronyms are what you're referring to. Indeed, flammable/inflammable, also sanction/sanction (permit/punish) and other examples such as fast/fast (going quickly/held in place). Still, I do find "I could care less" to be less of a contronym and more of an "Americanism". I'm quite used to it by now, and shall thereby sanction its use.