Definition of "vow"

And Moses spake vnto the heads of the tribes, concerning the children of Israel, saying, This is the thing which the Lord hath commanded.If a man vowe a vow vnto the Lord, or sweare an othe to bind his soule with a bond: he shall not breake his word, hee shall doe according to all that proceedeth out of his mouth.

1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], Numbers 30:1–2

Swearing doesn't just mean what we now understand by "dirty words". It is entwined, in social and linguistic history, with the other sort of swearing: vows and oaths. Consider for a moment the origins of almost any word we have for bad language – "profanity", "curses", "oaths" and "swearing" itself.

2013 June 14, Sam Leith, “Where the profound meets the profane”, in The Guardian Weekly, volume 189, number 1, page 37

There are also waxen vows, that represent other parts of the body mixed with them; but of these there are few in comparison of the number of the Priapi.

1786, Richard Payne Knight, The Worship of Priapus

When thou vowest a vow unto God, defer not to pay it.

1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], Ecclesiastes 5:4

We do not vow that we will never sin, nor neglect a duty (nor ought we to do so).

1673, Richard Baxter, Christian Directory