The noun cootie, denoting a body louse, first recorded in 1917, originated in army use on the Western Front during the First World War.
Two related words, the noun coot, meaning louse, and the adjective cooty, meaning infested with lice, are attested earlier, in 1915, but this does not necessarily mean that they predate cootie, which might have been in oral use before its first attestation.
The noun coot is first recorded in a letter dated 7th September 1915 that the American actor and playwright Harold Chapin (1886-1915) wrote to his wife:
Loud cries—Willet very pale and excited grappling with an enormous “coot” (otherwise louse). He makes a point of looking supremely surprised whenever he catches one on himself as if he were immune.
Harold Chapin was also the first known user of the adjective cooty, in a letter dated 22nd April 1915 that he wrote to his mother:
Our section has taken over duties this week. Two of the four Corporals have celebrated the occasion by “going cooty,” otherwise declaring possession of one or more lice and being quarantined in the scaby ward.
The American author, actor, screenwriter and film producer Arthur Guy Empey (1883-1963), who volunteered in the British army in 1915, published an account of his war experiences, titled “Over the Top” by an American soldier who went (New York, 1917); in its appendix, Tommy’s dictionary of the trenches, Empey defined the plural noun cooties as denoting:
Unwelcome inhabitants of Tommy’s shirt.
It has been said that the words cootie, coot and cooty ultimately reflect a borrowing of either Maori kutu or its cognate Malay kutu, meaning louse (or a related word in another Austronesian language). But there is nothing in the early uses of any of these three words to make such an origin seem likely. However, the use of coot in the sense of louse in the unpublished diary of John Henry Eagleston (1803-84), who traded between Fiji and Manila in the 1830s, probably shows an isolated borrowing from Malay.
(Some of the various later borrowed forms of Maori kutu in New-Zealand English were probably influenced by familiarity with coot and cootie.)
The probable explanation is that these three words, cootie, coot and cooty, ultimately refer to the aquatic bird of the rail family called coot; the reputation of this bird for being lice-infested gave rise to the phrase as lousy as a coot, as the English poet, farm labourer and naturalist John Clare (1793-1864) remarked:
These birds are subject to lice which is so common to them that it has grown into a saying that any thing filthy is ‘as lousey as a coot’.
G. F. Northall also recorded as lousy as a coot in Folk-phrases of four counties (Gloucestershire, Staffordshire, Warwickshire, Worcestershire) (London, 1894).
Indeed, the coot louse has its own scientific name, as the English botanist and mycologist Mordecai Cubitt Cooke (1825-1914) wrote in One Thousand Objects for the Microscope (London & New York, 1869):
Coot Louse (Nirmus Fulicæ).—The old notion about the coot is true as far as parasites are concerned. They are truly plentiful.
detail from an illustration for
One Thousand Objects for the Microscope (1869), by M. C. Cooke
(8: the coot louse)