'lepton' originates from Greek, specifically the word 'λεπτόν (leptón)', where 'λεπτός (leptós)' meant 'thin, small'.

'lepton' came into scientific English via New Latin/Modern scientific usage; the Greek 'λεπτόν' (meaning a small or thin thing, also a small coin) was adopted into New Latin/modern scientific vocabulary and then into English to name the class of very small particles.

Initially it meant 'a small or thin thing' (and was used for a small coin); over time it evolved into the modern scientific meaning 'a class of very small elementary particles.'