In a cocked or raised position; tilted upward or to the side, as when a gun's hammer is raised or someone's head is tilted.
From 'cock' (an old word meaning to tilt or raise) combined with the prefix 'a-' (meaning 'in a state of'). The term has been used since Elizabethan English to describe things in an angled or raised position.
You'll see 'acock' in old literature and pirate stories—'the captain held his hat acock' means he wore it at a jaunty angle, and it gives that playful, devil-may-care feeling that writers wanted to convey!
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