Hyphenation
dou·ble dutchTurkish pronunciation
dʌbıl dʌçPronunciation
/ˈdəbəl ˈdəʧ/ /ˈdʌbəl ˈdʌʧ/
Etymology
[ 'd&-b&l ] (adjective.) 13th century. double (twice, i.e. more than) + Dutch
For sense 1:
The English, historically, found Dutch (albeit a fellow Western Germanic language) particularly difficult to understand so anything that was completely incomprehensible would be double Dutch, i.e. twice as hard as Dutch.