Brewer's: Shilly Shally
A corruption of “Will I, shall I,” or “Shall I, shall I.”
There's no delay, they ne'er stand shall I, shall I, Hermogenes with Dallila doth daily.
Taylor's Workes, iii.3 (1630).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894