"Something old" is the first line of a traditional rhyme which details what a bride should wear at her wedding for good luck:

Something old,

something new,

something borrowed,

something blue,

and a silver sixpence in her shoe.

The old item provided protection for the baby to come.

The item borrowed from another happy bride provided good luck.

The color blue was a sign of fidelity.

The sixpence — a silver British coin — was a symbol of prosperity or acted as a ward against evil done by frustrated suitors. The superstition is known since 1883 when it was attributed to the county of Lancashire.
 

An 1898 compilation of English folklore recounted that:

In this country an old couplet directs that the bride shall wear:— "Something old, something new, Something borrowed, something blue." "The something blue" takes, I am given to understand, usually the form of a garter, an article of dress which plays an important part in some wedding rites, as, for instance, in the old custom of plucking off the garter of the bride. "The something old" and "something blue" are devices to baffle the Evil Eye. The usual effect on the bride of the Evil Eye is to render her barren, and this is obviated by wearing "something borrowed", which should properly be the undergarment of some woman who has been blessed with children: the clothes communicate fertility to the bride.

Another compilation of the era frames this poem as "a Lancashire version", as contrast against a Leicestershire recitation that "a bride on her wedding day should wear—'Something new, Something blue, Something borrowed'...", and so omits the "something old". The authors note that this counters other regional folklore warning against the wearing of blue on the wedding day, but relates the use of the color to phrases like "true blue" which make positive associations with the color.
 

The rhyme can earlier be found in an 1876 edition of Notes and Queries,[4] and is called an "ancient custom" in another 1876 book, Bye-gones, Relating to Wales and the Border Counties.[5] This version is referenced as well in an 1871 short story, "Marriage Superstitions, and the Miseries of a Bride Elect", in The St. James's Magazine