Cosmo Lavish is a member of the long-established and 'honourable' (they've got old money, which gives anything an honorific even if the initial acquisition of said money was somewhat dubious) Lavish family; one of those long-entrenched upper crust families who, in theory at least, can contend to gain the Patricianship for one of their members. The nearest thing they have to a plausible candidate, at the moment, is Cosmo.
Educated at the Assassins' Guild (although he did not take the Black Syllabus), Cosmo, like other members of his family (including his sister, Pucrezia "Pucci" Lavish), holds a minor directorship at the Royal Bank of Ankh-Morpork, so is therefore not an ally of Moist von Lipwig when Moist assumes control of the bank. Moist, upon hearing that Cosmo never graduated as an active Assassin, wonders out loud if Cosmo had taken a note in from his mother to the Master to excuse him from "stabbing practice".
Cosmo's plan is to take over as Patrician and with that in mind he becomes obsessed with Vetinari; his mannerisms, his leadership style and his affectations all being copied so that he can play the part properly when he 'takes control'. He therefore favours black-coloured clothing and wears a goatee. However, as someone points out, black is only really stylish on people who are not carrying excess fat and a goatee beard only looks sinister on people with one chin. Moist feels that Cosmo's goatee is seriously misplaced and pities his barber. Cosmo also tries unsuccessfully to train one eyebrow to raise quizzically, à-la-Vetinari, as in his opinion this is the source of the fascination the man extends over people, and the reason people bow down - or quiver in fear, either of which responses has appeal to Cosmo. Unfortunately, the eyebrow trick is beyond him, and instead it appears that he has a manic tic. He is convinced however that once he learns the quizzical eyebrow trick the Patricianship will naturally follow. His fascination with Vetinari extends to other sinister ways. He pretends Heretofore, his manservant/secretary is "Drumknott" which is Vetinari's aide's name. He instructs Heretofore to collect artifacts of the Patrician - boots, a skullcap, his sword-stick. Heretofore uses this as an opportunity to feather his own nest persuading Cosmos that he can get more 'items' for a price even though, in fact, most of the items, with the exception of the skullcap, are actually not Vetinari's. Cosmo also has his own personal killer, the assassin Cranberry, to clean up any 'loose ends' from these schemes and Heretofore is under no illusions that Cranberry will visit him if he oversteps his boundaries and makes a mistake. Despite having everyone involved in his schemes killed, Cosmo predictably does not manage to prevent his plotting from coming to the omniscient attention of the Patrician; the major skill he lacks in his dream of becoming Vetinari is an all encompassing vision of what is going on around him. Cosmo's ultimate downfall is his desire to own a similar signet ring to Vetinari's. Heretofore has the ring made to Vetinari's hand size so that Cosmo does not suspect that it is a fake and execute Heretofore. It is therefore too tight and his finger loses its feeling and becomes gangrenous which drives Cosmo to madness. In addition, being made of Stygium, the ring has to be kept gloved in the dark or it becomes warm, "extremely" warm in sunlight. Ultimately, when Moist pulls of the glove and exposes the ring to sunlight, the heat burns the gangrenous finger off and saves Cosmo's life. cosmo is then committed for the foreseeable future to a room in the Havelock Vetinari Ward of the Lady Sybil Free Hospital. He is content to remain there, surrounded by other madmen who believe that they are Lord Vetinari too. Cosmo believes that he is safe from attempts on his life because no one would ever suspect that the 'real Vetinari' would be hiding amongst a bunch of Vetinari lunatic wannabees. While he recovers he joins and bests his fellow patients in the eyebrow raising contest.
The name 'Cosmo Lavish' is a reference to both Cosimo de' Medici, a wealthy Florentine banker of the 15th century and the "lavish" life style of the rich and famous. The name Medici means 'doctors' in Italian while obviously Vetinari connotes 'veterinarian' - another kind of doctor. Like the Lavishes in Ankh-Morpork, the Medicis were an important powerful family in 15th century Italy and Europe, marrying into and controlling key thrones in Italy, France and the Holy Roman Empire as well as producing a number of popes.
The Lavishes also have strong connections to the Borgias, of whom Cesare and his sister Lucrezia were the most notorious - Pucrezia a clear take off on Lucrezia. The Borgia family also had its share of poisoners.
Pucrezia's nickname "Pucci" is a nod to the contemporary Florentine fashion designer favored by the rich and stylish of the same name. Interestingly, Emilio Pucci's ancestors were another wealthy and powerful merchant family during the Renaissance who were firm political allies of the Medici.
Finally, Cosmo's use of the raised eyebrow is a reference to British actor Roger Moore, who made the character of James Bond his own by the strategic use of a single lifted brow to convey a variety of emotional ideas, such as surprise, determination, sophistication and savour faire as befits one trained as an Assassin/007 agent. Many actors have tried to imitate Moore (and Vetinari), but have succeeded in only making themselves look ridiculous rather than suave lady-killers.