plain flour

"plain flour" means a wheat flour with a medium gluten content, suitable for general use. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.

"plain flour" is informal English meaning a wheat flour with a medium gluten content, suitable for general use. It is mainly heard in British English (and Irish English). A more formal or neutral equivalent would be something like "a wheat flour with a medium gluten content" expressed in standard vocabulary. A common mistake is to assume "plain flour" can replace its standard equivalent in every register; keep it for spoken or casual situations and use the formal form in writing. If you are a B2 or C1 learner, it is useful to recognise this word when you hear or read it, but think carefully about whether the situation really calls for it before using it yourself.