Absolutely genuine, quite authentic, as in That laboratory test was simon pure; none of the specimens was adulterated. This expression comes from the name of a character in a play, Susannah Centilivre's A Bold Stroke for a Wife (1717), who is the victim of an impersonation but turns up in the end and proves that he is "the real Simon Pure."
n. a very hard taskmaster; a hard boss. (From the name of the slave driver in Uncle Tom’s Cabin.) Ask Simon Legree if I will be able to stop work and go home for breakfast now.