as it is used in this verse, means,

Impure; morally lewd;
demonically foul.

Wherefore come out from among them,
and be ye separate, saith the Lord;
and touch not the unclean thing;
and I will receive you,
and I will be a Father unto you,
and ye shall be my sons and daughters,
saith the Lord Almighty.

2 Corinthians 6:17-18