2d

"2d" is an old or dialect word meaning (in pre-decimalisation currency, never used for 'two new pence'). If your teen uses it, they have most likely picked it up from a book, video game, period drama, or a regional grandparent rather than a modern peer group. There is no cause for concern here; the word is largely out of contemporary teen usage. If it appears in a school text, it is being studied for vocabulary or historical context. You can treat it like any other archaic vocabulary item.

"2d" is obsolete or literary English meaning (in pre-decimalisation currency, never used for 'two new pence'). It is mainly heard in British English (and Irish English). A more formal or neutral equivalent would be something like "(in pre-decimalisation currency" expressed in standard vocabulary. A common mistake is to use "2d" in modern conversation; it will sound strange or theatrical. Recognise it in older texts, but use the modern equivalent when speaking. 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.