Dashiell's exact origin is uncertain, generally traced to a French surname possibly meaning "page boy" or attendant, though the connection is looser than for many older names. It's become associated instead with a spirited, adventurous character, largely through its use by a well-known 20th-century writer.
To unfasten the tongues Mercury must be in its house and it is efficient with those who Mercury rules. - Picatrix (Andalusia, ~1000.AD)