What does "lac" refer to in what we call the lac operon : -

The lac operon of E. coli, described by Jacob and Monod, is a set of genes that control the use of lactose (milk sugar).
It contains a promoter, an operator and three structural genes: z (beta-galactosidase), y (permease) and a (transacetylase).
Lactose (as allolactose) acts as the inducer: it binds to the repressor and switches the operon on, so the enzymes are made only when lactose is present.
So 'lac' refers to lactose.