For good measure
As an additional extra.
'Good measure' has been part of the language since the first English-speaking 'purveyor of fyne goodes' set up shop, and it just means 'an ample or generous quantity of that which is sold by measure'.
The first instance of the expression in print is found where many other first coinages originated, in John Wyclif's Middle English translation of the Bible, circa 1384, in Luke 6:38:
Thei schulen yyue in to youre bosum a good mesure, and wel fillid, and schakun togidir, and ouerflowynge; for bi the same mesure, bi whiche ye meeten, it schal be metun ayen to you.
[They shall give into your bosom a good measure, and well-filled, and shaken together, and overflowing; for by the same measure, by which ye mete, it shall be meted again to you.]
We might expect the extended term 'in good measure' to refer to an abundance of something. In fact, its rather the reverse. 'Measured' also means 'moderate; restrained' and if a person acts 'in good measure' they are being especially temperate in their actions. As it happens, Wyclif was also one of the first to put that meaning of 'good measure' into print, in a collection of sermons known as Controversial Tracks, circa 1400, which was directed at the clergy:
Ye shulden lyue on ye puple in good mesure as Paul biddin.
[You be sustained by the people in moderation, as St. Paul bids you.]
It wasn't until much later that the use of the phrase 'good measure' returned to its original 'ample' meaning. In the 19th century people began to express the idea of things being 'thrown in for good measure', that is, added as a complimentary extra portion. In 1811, the British mathematician Patrick Kelly wrote The Universal Cambist, which was an exhaustive study of the weights and measures in use in different parts of the world and a method of converting from one to another. In the notes on Swedish measurement he included:
Corn, and other dry commodities, are measured by Tunnor. The Tunne is divided into 32 Kappar. But to every Tunna of wheat 4 Kappar are allowed for good measure.
Before long, the expression 'for good measure' began to be used figuratively, that is, in circumstances where no actual measurement was taking place. An example appears in the May 1850 edition of the American magazine Littell's Living Age, in a report of a public flogging in California:
'Give him another for good measure' - 'Hit him again' - were the sounds which greeted his ears.
'For good measure' might appear to be linked to the 'Baker's dozen', as both phrases express the notion of a little extra being added above the absolute requirement. In fact, the two phrases aren't connected, 'Baker's dozen' being much older. While the extra that was added 'for good measure' was added willingly, the extra that made up a Baker's dozen was added under threat of severe punishment. In mediaeval England, being light in the loaves was as risky as being 'light in the loafers' was prior to the decriminalisation of homosexuality in the 1967 Sexual Offences Act.