thirty, n.¹ — Green’s Dictionary of Slang

thirty n.¹

[the notation – 30 – used orig. by printers and telegraphers to indicate the end of a story or despatch]

(US) the end.

1889		Kansas Times and Star 7 May n.p.: At midnight ‘30’ (the end) flashed out on a large floral ‘sounder’ and a majority of the guests [...] adjourned [DA].
1895		Funk’s Standard Dict. n.p.: Thirty, [...] among printers and telegraphers, the last sheet, word, or line of copy or of a despatch; the last; the end [DA].
1930		R. Whitfield Green Ice (1988) 70: How about drinking with me tonight, after ‘thirty’?
1947		Sooner Mag. Nov. 22/2: He had just written the editorial sign ‘30’ to the copy of Volume 20, No. 4, of the publication which he had founded and over which he had presided for 20 years [DA].