[punning ref. to the US Kelsey Wheel Company, founded in 1910 to produce automobile wheels. The need for nuts and bolts to be exceptionally tight fitting to preclude wobbly wheels gave rise to the saying]
[1930s+] (US Und.) used alone to imply the best, the absolute; used in phr. below as a superlative to mean ‘extremely’, ‘the most’.
cold as Kelsey’s nuts
(alsocolder than Kelsey’s ass)
[1960s+] extremely cold.
dead as Kelsey’s nuts
(alsodeader than Kelsey’s nuts)
[1930s+] (US) very definitely dead, out of favour; lit. or fig.
safe as Kelsey
[1910s+] extremely conservative.
tight as Kelsey’s nuts