Unclear; possibly an alteration of English diddle. Compare the use as an expressive nonsense syllable in often sexual songs and the spelling dil doul, found in e.g. The Maids Complaint for want of a Dil Doul, a song in the library of Samuel Pepys. Unlikely to be related to Italian diletto (“delight”) or Latin dīlātō (“to open up, spread”), often cited etymologies.