"Insert your hand into your pocket"—as-luk means: insert it into your pocket [meaning: your shirt]. "It will come out white without evil (disease)."
﴿اسلك يدك فِي جيبك﴾ اسْلُكْ؛ أَيْ: أَدْخِلْهَا فِي جَيْبِكَ [أَيْ: قَمِيصِكَ] ﴿تَخْرُجْ بَيْضَاءَ مِنْ غير سوء﴾.
