Cotton on to
To get to know or understand something.
The phrase 'cotton on to', with the above meaning, appears to be limited in usage to the UK and other countries that were previously part of the British Empire, notably Australia and New Zealand. In the USA, especially in the southern states, 'cotton to' is used, with the slightly modified meaning of 'take a liking to'.
As early as 1648, in a pamphlet titled Mercurius Elencticus, mocking the English parliament, the royalist soldier and poet Sir George Wharton used 'cotton', or as it was spelled then 'cotten', as a verb meaning 'to make friendly advances'. 'Cotten up to' and 'cotten to' were both used to mean 'become friendly with'. Whether this was as a reference to the rather annoying predisposition of moist raw cotton to stick to things or whether it alluded to moving of cotton garments closer together during a romantic advance isn't clear. John Camden Hotten, in his Slang Dictionary, 1869, opted for the former derivation:
Cotton, to like, adhere to, or agree with any person; "to COTTON on to a man," to attach yourself to him, or fancy him, literally, to stick to him as cotton would.
The number of citations that use 'cottening' in a courtship context and the use of the 'cottening up' variant would suggest the latter is more likely; for example, William Congreave's comic play Love for Love, 1695:
I love to see 'em hug and cotten together, like Down upon a Thistle.
The attaching of cotton strands to the bobbins of weaving looms is sometimes also cited as a source of 'cottoning on', but there appears to be no basis for that notion. None of the early citations of the phrase mention that context.
'Cottoning on' as we now use it derives from the meaning of 'attaching oneself to something', specifically an attachment to an idea that we haven't encountered before. It would seem to be a reasonable bet that at least one of the variants of this phrase would have been coined in one of the major English-speaking cotton producing regions of the world, for example India or the USA. Not so; which gives more credibility to the notion that this phrase has little to do with the cotton plant. 'Cotton to' was coined in the UK and the first widespread uses of 'cotton on to' were in New Zealand and Australia. The earliest example that I can find of this is from the New South Wales newspaper The Maitland Mercury & Hunter River General Advertiser, March 1883, reporting on a local horse race:
A lot [of backers] then cottoned on to Sahara, who was a strong favourite.
This citation was closely followed by a reference in a New Zealand newspaper The Wanganui Herald, June 1893:
The Kaierau forwards are just beginnng to cotton on to the passing game.
It seems that 'cotton to', 'cotton on to' all derive from the same root source, i.e. the verb 'to cotton'. In the UK and its antipodes it has settled down as 'cotton on to', with the meaning 'form an understanding of' and in the USA it is 'cotton to', with the 'take a liking to' meaning.
See also: the meaning and origin of 'cotton-picking'.