To become quiet and calm after a period of restlessness.(verb)
To establish a settled lifestyle, and especially to marry.(verb)
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Use "settle down" in a sentence
""If at the end of that time (3 or 4 years hence) I could find some fair damsel who was foolish enough to trust me," [he suggested hopefully,] "I think I would get-married, and settle down to sober work for the rest of my days in some small city where one could enjoy the advantages of a city and at the same time not feel as if lost.""
"Oscar has already made it through his party-guy years and is more inclined to settle down now with the right girl."
"That might mean leaving your partner to his duck lorange while you grab a Big Mac elsewhere, or it might mean you cleanse together and settle down for the next course as a couple."