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1.
It means to get married or commit to a monogamous relationship; probably the two most indicative words for the situation itself, born of a word that can mean "to move downward; sink, or descend," "to subdue," and "to conclude," and a word that means the "opposite of up."

"Man looks like Lester finally had to settle down and give up the player lifestyle...man, and settle isn't wrong!"

by Clint October 09, 2005
 
2.
used in heated situations
when ross is tring to sniff ur missus biffon,
you say "come on , settle down"
by catzo lungo May 26, 2003
 
3.
verb. seh-tul down - 1) to stop knocking some one's fork on the floor especially at a cracker barrel. 2) to stop laughing uncontrollably to the point of endangering people within the area.
"Colleen will you please settle down. My fork has hit the ground 6 times already!"
by Xwhite chocolateX January 09, 2010