To rule the roost means to be in a dominating position over others.

This phrase conjures up a picture of a cock lording it over a group of hens, i.e. a roost, in the farmyard, and appears to be similar to cock of the walk, which designates someone who dominates others within a group.

But to rule the roost dates from the mid-18th century, and is a folk-etymological alteration of to rule the roast, a phrase first attested in The Debate of the Carpenter’s Tools, composed around 1500:

Bot whatsoever ye [= you] brage or boste [= boast],
My mayster yet schall reule the roste.

To rule the roast gave rise to ruler of the roast, which was used for example by James Bell (floruit 1551-96) in Against Ierome Osorius Byshopp of Siluane in Portingall and against his slaunderous inuectiues An aunswere apologeticall: for the necessary defence of the euangelicall doctrine and veritie. First taken in hand by M. Walter Haddon, then undertaken and continued by M. Iohn Foxe, and now Englished by Iames Bell (1581):

You seeme to marueile much that I beyng a Ciuiliā, and exercized in pleadyng temporall causes would spend my tyme to knowe your mysteries. Truly you are herein somewhat to inquisitiue Osorius. For albeit I do professe the Ciuill Law, yet am I a Christian and desire to be edified in the law of the Lord: And if you will haue this much graunted vnto you, to apply your selfe to the knowledge of the tounges, to be addict wholy to the study of eloquence, to raunge in the bookes of Philosopers, and will notwithstanding be accompted a ruler of the Roast in Diuinitie, as in the speciall peculiar of your own profession.

Both to rule the roast and ruler of the roast were common from the mid-16th century onwards.

It is often said that to rule the roast originally referred to the joint of meat that would be carved by the master of the house, or be the principal dish at the table he presided over, i.e. ruled. It is alternatively said that it alluded to someone being the most important person at a banquet or feast.

However, several 17th-century examples indicate that the phrase may have arisen from the sway exercised by master cooks. In Micro-cosmographie, or, A peece of the world discovered in essayes and characters (1628), the English bishop John Earle (1601?-1665) wrote:

A Cooke.
The Kitchin is his Hell, and hee the Diuell in it, where his meate and he frye together. His Reuennues are showr’d downe from the fat of the Land, and he enterlards his owne grease among to helpe the drippings. Colericke hee is, not by nature so much as his Art, and it is a shrewd temptation that the chopping knife is so neare. His weapons ofter offensiue, are a messe of hot broth, and scalding water, and woe bee to him that comes in his way. In the Kitchin he wil domineere, and rule the roste, in spight of his Master, and Curses is the very Dialect of his Calling.

And in Microcosmus A morall maske (1637), the English dramatist Thomas Nabbes (1605-41) wrote:

I am my Ladies Cooke, and King of the Kitchin: where I rule the roast; command imperiously, and am a very tyrant in my office.

Women too could rule the roast: Gynaikeion: or, Nine bookes of various history. Concerninge women inscribed by ye names of ye nine Muses (1624), by the English playwright, actor and author Thomas Heywood (1574?-1641), contains the following:

A citisen of good reckoning hauing a faire wife, kept diuerse prentices and maid-seruants in his house: one of those busie young fellowes had cast a wanton eye vpon her that ruled the rost in the kitchin.

Frederick William Hackwood seemed to confirm this origin in Good Cheer: The Romance of Food and Feasting (1911):

The office of the culinary artist is one which has a subtle, if not a dominating, influence on the daily life of those for whom it is his business to cater. He rules the roast,  says one proverb; the cook is not to be taught in his own kitchen, says another.

To rule the roast was the only version of the phrase entered in the first edition (1914) of the Oxford English Dictionary, and the version with roost was not even added in its 1933 Supplement. Presumably, the earlier version having become obscure, it was progressively replaced by to rule the roost, which has greater immediacy. But, as Robert Allen writes in Allen’s Dictionary of English Phrases (2008),

‘ruling the roast’ might now conjure up a picture from the world of suburban barbecues.

This advertisement from the Surrey Advertiser and County Times of 14^th January 1939 seems to indicate that to rule the roast was still in usage at that time:

Rules the Roast
ARGENTINE
CHILLED
BEEF
ALWAYS IN PRIME CONDITION