Delete words from the built-in dictionary?

Using Android 6. Auto correct usually works very well, so I don't want to turn it off. However on some occasions it stubbornly 'corrects' some words which I don't want, which is *extremely* inconvenient. These words are not in my personal dictionary. I've tried the following: - add the word I want to my personal dictionary - add the word I want to the 'all languages' dictionary - add the wrong word (to which it tends to autocorrect) to the custom dictionary and then remove that - repeatedly type in the word and when it autocorrects, click it and set it back to what it was, hoping that Android will learn my preference (I really tried this many many interations) It's driving me nuts. How do I force Android NOT to autocorrect certain words? Or rather, how do delete some words from its own dictionary? P.S. I know several similar questions like these were posted before, but they were all either outdated (apparently referring to much older Android versions) or the posted answers didn't work (I've tried them all). Quite some references to Android in 2011 are no longer relevant. P.P.S. I'm using a Nexus 5, default built-in keyboard.