WordNet sense 1 (the illegitimate offspring of unmarried parents):

by-blow, whoreson, illegitimate child

WordNet sense 2 (having no legally established claim):

unlawful

WordNet sense 3 (born out of wedlock):

misbegotten, spurious

WordNet sense 4 (illegitimate):

base, baseborn

WordNet sense 5 (contrary to or forbidden by law):

illicit, outlaw, outlawed

WordNet sense 6 (prohibited by law or by official or accepted rules):

illegal