Every learner encounters new words. If each unknown term were automatically judged non-fasih, an individual student's vocabulary gap would wrongly become a linguistic standard.
Gharaba must be assessed against reputable Arabic usage and the linguistic environment of the original expression. A term may be established in a specialized context but unusual in ordinary speech.
Conclusion: Looking up an unfamiliar word is appropriate; one's personal lack of familiarity does not, by itself, establish technical gharaba.