1540s, from Fr. adopter
(14c.), from L. adoptare
"choose for oneself" (esp. a child); see adoption
. Or perhaps a back-formation from Eng. adoption
. Originally in Eng. also of friends, fathers, citizens, etc. Sense of "to legally take as one's own child" and that of "to embrace, espouse" a practice, method, etc. are from c.1600.