Frege's context principle, picked up by Wittgenstein, goes like this:
"Never ask for a meaning of a word in isolation, but only in the context of a proposition"
Although it has the force of an 11th commandment, the "never" in Frege's idea points to his failure to understand the whole situation that he is speaking about, and so he can only validate the rule that he wants to draw from it by using an injunction.
So let me fill in for Frege by first pointing out what probably goes without saying: words never have a meaning, but we give marks a meaning.
The missing idea in Frege's Context Principle must surely be this:
Words have meanings in a context, and where there is only a word (or rather the sign for one) then a context is mentally seconded to it.
That is, words, even single words, always occur in the context of a proposition. It just behoves us to be more specific as to what proposition the "word in isolation" is alluding to.