invaluable means "cannot be valued" meaning just like indestructible means "cannot be destroyed." Colloquially, when we say something can't be valued we mean the thing is so important that you literally can't conceive of a finite value you could assign to it. More on reddit.com

In 3rd grade I used a thesaurus to find a synonym for "running away". My teacher was pretty surprised when my characters eloped... One of my fondest memories tbh More on reddit.com

AFAIK this is a 2 part response. In Latin, the prefix "in-" was a negator on nouns (like illogical) and an intensifier on verbs (like indoctrinate). Sometimes we derive nouns from verbs (or vice versa) and the prefix's meaning gets funky. The second part is just semantic drift. Basically, sometimes words start to diverge in meaning and sometimes they begin to mean the same thing. For example, "awesome" used to mean "awe inspiring" with a scary connotation, but it began to semantically drift and now just means something that is very interesting or very cool. There are others, but I don't know them off the top of my head, but I hoped this (at least partially) answered your question. :) More on reddit.com

That's because worthless and worth full are antonyms More on reddit.com