Descriptive of the increasingly large number of people who do not conform to the traditional values of man + woman = ok. Modern
People tend towards being attracted to their own sex, or even both, in which case they are considered to be Thoroughly Modern People.
Symptoms of Modernity include overly tight/baggy clothes (boys/girls respectively), visible underwear, and publically being amorous with a member of the same sex.
Term was initially invented in the Birmingham area of the UK as a means of talking about suspected Modern People in close proximity without the subjects being aware of what was being discussed. Its growing popularity is rendering it useless in this regard.
See also: Modern People, Special Friends, or the short forms MP and SF