(n.) disorganization, mismanagement
Rack-and-manger is total disorganization—while ‘to go to rack and manger’ is to be ruined through lack of good management, or improper use.
As some of the comments that term rightly worked out on Twitter, there’s a link here between this and the more familiar expression rack and ruin—but the rack is different in each one.
In rack-and-manger, we’re talking agricultural equipment: a manger of course is a feeding trough for livestock, while a rack is a similar wire-frame feeding trough, often filled with hay and fastened to a wall. In this sense, rack is a decidedly Germanic-origin term (and probably originally meant simply ‘framework’ or ‘mesh’), while manger is derived from French and Latin (and unsurprisingly is related to manger, the French word for ‘to eat’).
In rack and ruin, the rack is different. Here, it’s a (probably originally dialect) variation of wrack, which is itself related to words like wreck and wreak. All these have their roots in wrecan, an Old English word meaning avenge or punish.
Of the two expressions, it was rack and ruin that came first: probably coined alliteratively, for rhetorical effect, it dates from the 1500s. Rack-and-manger dates from the late 1600s: the Oxford English Dictionary has evidence of an older form, ‘to leave all at rack and manger’, which was given as the English equivalent of the French phrase laisser tout à l’abandon in a bilingual dictionary published in 1687.
Rack and manger is ultimately little more than a later play on the better established rack and ruin, bringing two otherwise unrelated items of farmyard equipment together for a jokey extension of the earlier phrase.