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: cf. 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."