Gey adjective considerable; adverb considerably, very
‘“It will be gey hard work,” Jamie had said.’ (Drums of Autumn)
The Older Scots spelling gay gives us a clue about this word’s origins. From the fourteenth century, the adjective gay meant splendid or cheerful (as in English), but in 1686 the modern Scots sense appears in George Stuart’s A Joco-Serious Discourse in two Dialogues between a Northumberland Gentleman and his Tenant a Scotchman: ‘Any of your enemies (of which ... yo’ve [sic] had a gay convenient number)’.
Walter Scott uses it as an adjective with the older spelling but new meaning in Guy Mannering (1815): ‘Kippletringan was distant at first, “a gay bit”. Then the “gay bit” was more accurately described, as “aiblins [perhaps] three mile”’. In the Laird of Logan (John Donald et al.), we find this fanciful description of penury: ‘[Hornie - a nickname for the Devil] micht hae made his cloots clatter a gay while in the bottom o’ your pouches, before he wad hae skinned his kutes (ankles) on bawbee or bodle that was there’.
A shadow of the older meaning survives where it can be interpreted as fine or excellent, and this is often intended ironically. From Grassic Gibbon’s Sunset Song (1932) we have ‘And Ellison had begun to think himself a gey man in Kinraddie’.
Gey often combines with ‘and’. James Hogg uses it in Tales of The Wars of Montrose (1835): ‘I hae a gayin muckle wallet fu’ o’ gowd that has never seen the light yet’. And from Neil Munro in The Daft Days (1907): ‘Reading and writing, and all the rest of it, are of less importance, but I’ll not deny they’re gey and handy’. (Compare ‘good and well’.)
There is some understatement in John Firth’s Reminiscences of an Orkney Parish (1922): ‘By midnight most of the men, at least, would be “gey weel tae live”, as they mildly expressed the earlier stages of intoxication.’ In some dialects, the pronunciation has changed too, more like ‘pie’ rather than ‘day’, but, however you pronounce it or spell it, it is a gey useful word.