WordNet sense 1 (half the width of an em):

en

WordNet sense 2 (a whimsically eccentric person):

crackpot, crank, fruitcake, screwball, nutcase

WordNet sense 3 (someone who is so ardently devoted to something that it resembles an addiction):

addict, freak, junkie, junky, addict

WordNet sense 4 (one of the two male reproductive glands that produce spermatozoa and secrete androgens):

ball, egg, testicle, testis, ballock, bollock, orchis