You may be familiar with the term “false friends”, those words which appear to mean the same thing in two different languages, but in fact mean something different. However, it is less likely that you have come across the term “false beginners” unless you have worked in language teaching.
The “false” beginner is the person who claims to be a complete beginner in a language, but in fact already has some knowledge. This knowledge may be hidden in the recesses of memory and therefore not easy to recall, it might be passive knowledge that has been picked up some time in the past but never put to active use, or it might be the case that the person concerned lacks the confidence to admit to any knowledge of the language and so refers to himself or herself as a complete beginner, when this isn´t strictly the case. I’m sure you have come across Spanish people who claim not to speak any English, or to speak very little, only to find they are practically fluent. Often the person has made this claim not to deceive anybody, but just through lack of confidence in their own abilities.
This phenomenon is quite common, but if you claim to be a beginner when strictly speaking you are not, you can create a dilemma for a language teacher. However rusty, badly remembered or impractical your knowledge of a language might be, there is a world of difference between you and a person who has truly never come across a language before. People who are learning a language for the very first time will take far longer to understand, assimilate and remember new words and concepts, whereas a “false beginner” usually only needs to have their memory refreshed once or twice for the same expressions to stick. This difference in speed of learning can create problems in the classroom, depending on the attitude of the student. If a “false beginner” is happy to keep quiet when necessary and be patient, they can gain from the class without intimidating others, but someone who says they are a beginner but who then proceeds to demonstrate knowledge beyond the rest of the group, and who also learns far more rapidly, is inevitably going to intimidate others in the class.
One of the problems with the expression “false beginner” and the reason you may never have heard it, is that although it is an accurate description of a learner from the point of view of the teacher, it does rather sound as though the person is being in some way deceitful. I have rarely made the mistake of “accusing” someone of being a “false beginner” as they are likely to take offence and go into long explanations of how little they actually know! In the academy I first worked in, we politely named a group as “post-beginners” which is actually a little more advanced than “false beginners” but covers the situation without hurting anyone’s feelings.