From
www.hmsrichmond.org :
RABBITS
Naval slang name given to articles taken, or intended to be taken, ashore
privately. Originally "rabbits" were things taken ashore improperly (i.e.
theft or smuggling - the name arose from the ease with which tobacco, etc.,
could be concealed in the inside of a dead rabbit) but with the passenger of
time the application of the word has spread to anything taken ashore; an air
of impropriety nevertheless still hangs over the use of the word, whether or
not this is justified (it seldom is). Hence the phrase "Tuck its ears in",
often said to an officer or rating seen going ashore with a parcel.
G