That phrase is not used in the Bible. Here's what Paul states:
Rom 5:12 (NAS) Therefore, just as through one man (Adam) sin entered into the world, and death through sin, and so death spread to all men, because all sinned (in Adam)....
Through Adam, sin entered the human race. Sin (singular), the love of self (iniquity), polluted the human race within Adam. Since our life substance was in Adam when he transgressed, all Adam could pass pass to his posterity, through procreation, was a life polluted with iniquity.
As I have stated before, iniquity is not an act (as in transgression), but rather an condition. When Adam transgressed his nature took a u-turn from agape love to self-love. Because our lives are the multiplication of Adam's fallen life we are born in iniquity, i.e., we are born with self-love (not agape). Since we are born without God's Spirit dwelling in our spirit (i.e., our inner mind) our fallen nature and mind are naturally in harmony and we instantly sin.
King David, as an infant, is a good example:
"Behold, I (David) was brought forth in iniquity, And in sin (iniquity) my mother conceived me." Ps 51:5 (NAS)
As a result of the fall, man by very nature is spiritually “bent,” so that the driving force of his very nature is the love of self. Paul defines it as “the law of sin and death” [Romans 7:23; 8:2]. It is this condition that is the basis of all our sinning, and which makes us slaves to sin [Romans 3:9-12; 7:14].