E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Cuckoo - Spit,
Frog - Spit, or Froth-Spit. The spume which forms the uidus of an insect called the Cicada Spumaria, or, more strictly speaking, the Cercopis Spumaria (one of the three divisions of the Cicadariæ). This spume is found on lavender-bushes, rosemary, fly-catch, and some other plants. Like the coohineal, the cicada spumaria exudes a foam for its own warmth, and for protection during its transition state. The word cuckoo in this case means spring or cuckoo-time.