WordNet (r) 3.0 (2006):

settle down
    v 1: settle into a position, usually on a surface or ground;
         "dust settled on the roofs" [syn: settle, settle down]
    2: become settled or established and stable in one's residence
       or life style; "He finally settled down" [syn: settle,
       root, take root, steady down, settle down]
    3: become quiet or calm, especially after a state of agitation;
       "After the fight both men need to cool off."; "It took a
       while after the baby was born for things to settle down
       again." [syn: calm, calm down, cool off, chill out,
       simmer down, settle down, cool it]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "settle down":
   alight, anchor, attain majority, be conservative, be moderate,
   be pacifistic, billet at, bivouac, bloom, burrow, camp, cave,
   cave in, colonize, come down, come in, come of age, come to anchor,
   come to maturity, crash-land, decline, descend, develop, ditch,
   domesticate, downwind, droop, drop anchor, ensconce,
   establish residence, fledge, flop, flop down, flower, flump,
   flump down, founder, give way, go down, grow, grow up, hive,
   inhabit, keep house, keep the peace, keep within bounds,
   keep within compass, land, lapse, leave the nest, level off, light,
   live at, live temperately, locate, lower, mature, mellow, moor,
   move, nest, not make waves, not resist, overshoot, pancake, park,
   people, perch, plop, plop down, plump, populate,
   practice nonviolence, practice self-control, reach manhood,
   reach twenty-one, reach voting age, relent, relocate, remit,
   reside, ripen, roost, sag, season, set, set up housekeeping,
   set up shop, settle, sink, sink down, sit down, slouch, slump,
   slump down, sober down, squat, stand, stay at, strike a balance,
   strike root, submerge, subside, swag, take in sail,
   take residence at, take root, take up residence, talk down, temper,
   toga virilis, touch down, upwind