dil, n.³ — Green’s Dictionary of Slang

dil n.³

also dilly

[despite cite 1926 ? weakened descendant of dell n.]

(US) a girlfriend.

1926		Judge (NY) 91 July-Dec. 31: Dil or Dilly - This word is used a great deal by the 'collitch' boys. It is of Arabic origin and refers to the female companion of a date.
1934		The Manet (N. Quincy High School, MA) n.d. 55: What words do you fellows use when referring to the fair-sex? [...] ‘Dill,’ ‘Blister,’ ‘Coozy,’ ‘Frail,’ ‘Femmie,’ [etc].