Middle English setlen, "become set or fixed, stable or permanent; seat, place in a seat; sink down, come down," from Old English setlan "place in a fixed or permanent position; cause to sit, place in a seat," from setl "a seat" (see settle (n.)). Compare German siedeln "to settle; to colonize."
From c. 1300 in reference to birds, etc., "to alight." From early 14c. of ground, etc., "to sink down, descend; cave in." By early 15c. (Chauliac) in reference to a liquid, "change from a disturbed or muddy condition to one of cleanness." By 1570s, of persons, "change from a disturbed or troubled state to one of security."
It is attested by 1520s as "become calm" (but c. 1600 it also could mean, colloquially, "knock down dead or stunned"). The meaning "decide, set or fix as by purpose or intention" is by 1620s. The meaning "secure title to (property, etc.) by means of a deed, etc." is from 1660s. It is attested by 1733 as "put beyond dispute or establish by authority or argument;" hence "resolve, determine, come to a decision (1782).
The sense of "establish a permanent residence" is recorded by 1620s; that of "plant with inhabitants, colonize" is by 1702.
The old meaning "reconcile" (a quarrel, differences, etc.) perhaps is influenced by or merged with Middle English sahtlen "to reconcile," which is from Old English saht "reconciliation," from Old Norse satt "reconciliation."
To settle down (intrans.) as what married couples do in establishing a domestic state is by 1835 (settle alone in this sense is by 1718). The transitive sense is by 1520s. To settle for "content oneself with" is from 1943; Middle English also used settle (v.) in an intransitive sense of "come down in the world, become lower in estate" (mid-14c.).