Lashings is UK slang for 'lots and lots'. The canonical examples are "lashings of butter" and "lashings of cream", which may give a hint at the reasoning behind the slang -- you lashed on the spreads with wild abandon.
Perhaps the best know use of the term is from the film Five Go Mad in Dorset, which referred to "ham and turkey sandwiches, bags of lettuce, hard-boiled eggs, heaps of tomato, and lashings of ginger beer", which has come to be strongly associated with the Famous Five. As it happens, Enid Blyton never did use the phrase "lashings of ginger beer", although the Famous Five did refer to lashings of eggs, snakes, and treacle. The result is that one of the most familiar memories people have of their favorite childhood books actually comes from a somewhat perverted comedy film with no connection to the author.