valuable

Many people find it confusing that the in- prefix at the beginning of invaluable apparently lacks the meaning "not" found in a number of other words, such as invalid, inarticulate, and insane. In fact, the prefix does indicate negation, but in a way that is not immediately obvious. The original (and current) meaning of invaluable is "valuable beyond estimation"; the word describes something so precious that one cannot assign a price to it. This, clearly, is the opposite of the meaning "having no value; valueless" that the word might seem to carry. Invaluable actually has been recorded in the sense "without value," but such use has been exceedingly rare and is practically nonexistent today.