Since there are so many words missing in English (and not just American English), the common words are given the task of having multiple meanings. How does one know which meaning to apply? The common answer is context.
With today's communications, there's often no context. Text is often squeezed into tiny spaces (cell phone screens, button captions, road and other signs). It's easy to put together two or three words, all of them loaded with multiple meanings, and divine their intended meanings from all the possible combinations of contexts.