1
general::
Ask for or expect without any prospect of success, as in If you want a cash refund, you can just whistle for it. [Mid-1700s]
American Heritage Idioms
2
general::
Phrase(s): whistle for someone or something
to summon someone or something by whistling. • I stood on the corner and whistled for a cab, but they all ignored me. • I whistled for the dog, but it did not appear.
McGrawhill's American Idioms And Phrasal Verbs