1550s, endearing name for a female pet or a mistress; originally a familiar form of fem. proper name Dorothy (q.v.). The -l- for -r- substitution in nicknames is common in English: compare Hal for Harold, Moll for Mary, Sally for Sarah, etc. Attested from 1640s as colloquial for "slattern;" sense of "child's toy baby" is c. 1700. Transferred back to living beings 1778 in sense of "pretty, silly woman."
1867, "to pet, indulge," from doll (n.). Usually with up. Meaning "to dress up" is from 1906, American English. Related: Dolled; dolling.