Green’s Dictionary of Slang

number ten adj.

also number sixty-nine
[pidgin, imported by veterans of the wars in Korea (1950–3) and Vietnam (1964–75)]

(orig. US milit.) very bad, the worst.

[UK]True Apr. 39: From the American occupation of Japan, the phrase ‘number one’ (from the Japanese ‘ichiban,’ meaning very good, the best) found a permanent place in GI talk. By extension, ‘number ten’ came to mean very bad, the worst.
[US]Army Reporter Feb. in Maledicta VI:1+2 251: I know you numbah fuckin’ ten cheap charlie.
[US]L. Heinemann Paco’s Story (1987) 178: GI beaucoup number ten goddamned shit-eating fucking pig.
[US](con. c.1970) G. Hasford Phantom Blooper 23: Life is real hard out here in the bad bush. We pulling a definite number-ten hump.
[US](con. 1964–8) J. Ellroy Cold Six Thousand 331: Fuck you. You cheap Charlie. American Punk No. 10.