The love of youthful pleasure. Chaucer uses the word “coltish” for skittish. Horses have at three years old the colt's-tooth. The allusion is to the colt's teeth of animals, a period of their life when their passions are strongest.
Her merry dancing-days are done; She has a colt's-tooth still, I warrant.
King: Orpheus and Eurydice
Well said, Lord Sands; Your colt's-tooth is not cast yet.
Shakespeare: Henry VIII., 1.3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894