Green’s Dictionary of Slang

flatfoot v.2

[? after drinking one slaps the glass flat on the table or bar]

(US) to down a glass of liquor in one gulp.

G.L. Coon Short End 234: Because he flatfooted all I had left in the jug, about a quarter of a pint, which might not sound like much, and might be more impresssive if I told you I would just as soon flatfoot so much lighter fluid [DARE].