Green’s Dictionary of Slang

chief, top, most important.

c.1535		implied in cock lorel n. (1)
1676		Etherege Man of Mode II i: Why the very cock-fool of all those fools, Sir Fopling Flutter.
1845		R.S. Surtees Hillingdon Hall III 93: First [...] is the great cock offender, called ‘incorrigible rogue’ a chap wot’s too bad for anything.
1857		T. Hughes Tom Brown’s School-Days 342: They’ll make the old Madman cock medicine-man and tattoo him all over.
1911		Gem 21 Oct. 8: We’re the cock-house at St. Jim’s!
1970		Current Sl. IV:2 5: Cock, adj. Attractive; ‘cool’; desirable.