Lupin seed is used to enhance the verility of rams in agriculture. Apparently, it can increase the size of the testes up to four times. It is supposed that a gentleman with extremely well developed testes could be suspected of consuming lupin seed, or may just be called a lupin seed eater.
Jane was discussing her night with Jeremy to her sister, She commented on his genetalia stating that he must be eating lupin seeds. His testes were quite sizeable in nature, too big to be natural. Her sister commented, "Yeah, definitely a LUPIN SEED."