Heads; so called from their resemblance to nuts. Probably “crack,” applied to heads, is part of the same figure of speech.
To go off their nuts about ladies, As dies for young fellars as fights.
It is time to lay our nuts aside (Latin, Relinquere nuces). To leave off our follies, to relinquish boyish pursuits. The allusion is to an old Roman marriage ceremony, in which the bridegroom, as he led his bride home, scattered nuts to the crowd, as if to symbolise to them that he gave up his boyish sports.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894