The genitalia (Not the testes, but the penis) of a man. "Johnsen" refers to the actual penis of the man, not the crotch, the testicles, or any other region.
If the person in question has several "Johnsen"s, then this person is referred to as "Johnsenrific".
"Look there, my children! That's my Johnsen!"
one who is fasinated with the male genitalia
wow look at that fag he is such a johnsen