1. [mid-19C+] (US) a proper or square meal [abbr.].
2. [mid-19C–1960s] (Aus., also square gin) gin, a bottle of gin [the shape of the bottle].
3. in fig. senses, implying gullibility, innocence, a failure to fit in with group norms [American Dialect Society (Nov. 1958) suggests the steady 1-2-3-4 rhythm played without variation].
(a) [mid-19C] (UK Und.) a respectable pose; an excuse, thus a shield from arrest.
(b) [20C+] (orig. US) in general use, a respectable working man or woman .
(c) [1940s+] (US) a respectable woman.
(d) [1930s+] (orig. US black) one who is lacking in sophistication or ‘cool’ as judged by the norms of a given peer group.
(e) [1940s–60s] (US drugs) one who eschews drugs.
(f) [1940s+] (US black) a man who pursues women and is thus tricked out of his money.
(g) [1950s+] (gay) a heterosexual, or a gay man who is not used to the gay scene.
4. [1920s+] (US, usu. prison) a factory-made cigarette, whether prison-issue or commercially produced [square adj. (1)].
5. [1970s] (US black) a sexual deviant.
6. [2010s] (UK Black) a credit card.
[1910s–60s] (Aus.) a sexually experienced woman, positioned in the contemporary moral spectrum between an all-out prostitute and a respectable woman; an ‘amateur’ prostitute.
SE in slang uses
[1950s+] military drill.
[1960s+] (orig. US) the starting point, the beginning.
[2000s] (US prison) to prepare for a fight.
[1960s] (US) in the respectable world.
[1930s–40s] (US black) a misfit.
[2000s] (US prison) to call out to have a fight.