To get married. This expression dates from the sixteenth century, or rather, is an abbreviation of one used then. It originally was to tie a knot with one’s tongue that one cannot untie with one’s teeth, and so appeared in several earlier printed sources as well as in John Ray’s 1670 proverb collection. The analogy is clear: the bonds of marriage are viewed as a knot, which, were it of string or cord, could be undone with the teeth—in other words, an early mixed metaphor. Although the full saying still appears in Rustic Speech, a collection by E. M. Wright published in 1913, all but “tie the knot” had long been dropped and survives as the current cliché, although in this age of relatively common and simple divorces it may be obsolescent.