A stance, with the hands on the hips and elbows directed outwards.
'Akimbo' is one of those odd words, like the 'aback' of 'taken aback' and 'fell' of 'one fell swoop', that is rarely used other than in its customary phrase - although there has been a spate of uses of 'legs akimbo' recently, since the UK comedy team The League of Gentlemen used that as the name of a spoof theatre troupe.
The 'arms akimbo' stance generally indicates that combination of impatience and defiance which is these days called 'attitude'. That is well exemplified in this design for a statue of the US politician Barbara Jordan.
'Akimbo' began life as 'kenebowe' and the first known recording of it is in the Tale of Beryn, circa 1400, or The Second Merchant's Tale. This piece, by an unknown author, was added to a 15th century edition of Chaucer's Canterbury Tales:
"The hoost... set his hond in kenebowe."
The spelling didn't settle down until the 20th century. In the preceding centuries, when the need for standardised spelling wasn't keenly felt, many variants were used - kenbow, kemboll, a-gambo, kimbow, akembo - anything that sounded like kenebowe.
How 'kenebowe' derived isn't clear. Some scholars have suggested that it originated as the Icelandic 'keng-boginn', i.e. 'bent in a horse-shoe curve'. Others have put forward the Middle English 'cam bow', i.e. 'crooked bow'.