Nach Strich und Faden means thoroughly, completely, in every respect. Sie hat ihn nach Strich und Faden betrogen. She deceived him thoroughly. The Duden traces the phrase to the language of weavers. When a journeyman’s piece was examined, the examiners checked the woven thread, the Faden, and the direction of the weave, the Strich.
The everyday word for thoroughly comes from the examination of a weaver’s journeyman piece.