curd: [14] Curd began life as crud, a word which has survived in its own right. In the 15th century it underwent a process known as metathesis, by which the sounds r and u became transposed, producing curd. A derivative of this, dating from the 16th century, is curdle. The word’s ultimate ancestry is not known, although some consider that Gaelic gruth may be related.
curd (n.)
c. 1500, metathesis of crud (late 14c.), originally "any coagulated substance," probably from Old English crudan "to press, drive," from PIE root *greut- "to press, coagulate," perhaps via ancestor of Gaelic gruth (because cognates are unknown in other Germanic or Romance languages).