违法
非法
不正当

[ noun ]
the quality of not conforming strictly to law
<noun.attribute>

Illicit \Il*lic"it\, a. [L. illicitus; pref. il- not + licitus,
p. p. of licere to be allowed or permitted: cf. F. illicite.
See {In-} not, and {License}.]
Not permitted or allowed; prohibited; unlawful; as, illicit
trade; illicit intercourse; illicit pleasure.

One illicit . . . transaction always leads to another.
--Burke.
-- {Il*lic"it*ly}, adv. -- {Il*lic"it*ness}, n.