Tie The Knot Meaning

(idiomatic, intransitive) To marry, wed, get married.

Example: 1869, William Makepeace Thackeray, Burlesques, ch. 3,
  Rowena sacrificed her inclination to remain single, to her sense of duty; and contracted a second matrimonial engagement. . . . Cardinal Pandulfo tied the knot for them.
2002, Scott MacLeod, "Live from Baghdad," Time, 25 Nov.,
  After the 20-year-old mechanic tied the knot with his 18-year-old cousin, the newlyweds headed to Baghdad's Ishtar Hotel.
Used other than as an idiom: see tie,‎ the,‎ knot.