There's a argument frame for some English verbs:
(Sbj, Obj, PP_of)
- Thomas rid the town of beggars.
- Thomas cured the boy of lupus.
- Thomas cleared the field of trees.
of has ablative use, and the template is normally restricted in meaning to a case where an undesirable thing is taken away. When an idiom gets grammaticalized, it's always best to use an available argument frame so people can pick up on the meaning easily.
The hanging speech of Lady Jane Grey, given April 12, 1554 is an early attestation in print:
GOOD Christen people, I am under a lawe, and by a lawe I am condempned to dye;
not for ani thing I have offended the Quene's Majesti, for I wil wash my hands giltles therof, but only for that I consented to the thing whiche I was inforced unto. Notwithstanding, I have offended Almyghtie God, for that I have folowed over much the lust of mi flesh, and the pleasure of this wretched world, and I have not lived according to the knowledge that God hath geven me; wherfore God hath plaged me nowe wyth thyskinde of death, and that worthelye, accordyng to my desertes. Howebeit, I thancke him hartelie, that he hath geven me time to repent my syns here in this world, Wherfor, good Christian people, I shal desyre you al to pray with me, and for me, while I am now alive;
that God of his goodnes will forgeve me my sinnes. And I pray you al to beare me
witnesse, that I here dye a true Christian woman, and that I truste to be saved by the bloud of Jesus Christ, and bi none other meanes ; and now I pray you al, pray for me, and with me!" and so saied the Psalm of Miserere mei : that don, she saied, " Lorde save my soule, whyche now I commend into thy handes :" and so prepared her selfe meekelie to the blocke.