c. 1200, o cros, "in the shape of a cross;" c. 1300, a-croiz, "in a crossed position;" early 14c., acros, "from one side to another;" a contraction of Anglo-French an cros, literally "on cross;" see a- (1) + cross (n.)).
Meaning "on the other side (as a result of crossing)" is from 1750. In crossword puzzle clues from 1924. Spelling acrost, representing a dialectal or vulgar pronunciation, is attested by 1759. Phrase across the board "embracing all categories" (1945) is said to be originally from horse-racing, in reference to a bet of the same amount of money on a horse to win, place, or show. To get (something) across "make (something) understood or appreciated" is by 1913, probably from earlier theater expression get (something) across the footlights, perform it so as to be received by the audience (1894).